Highly conductive graphene foams and process for producing same

ABSTRACT

A process for producing a solid graphene foam composed of multiple pores and pore walls The process comprises: (a) preparing a graphene dispersion having a graphene material dispersed in a liquid medium, which contains an optional blowing agent; (b) dispensing and depositing the graphene dispersion onto a supporting substrate to form a wet layer of graphene material having a preferred orientation; (c) partially or completely removing the liquid medium from the wet layer of graphene material to form a dried layer of graphene material having a content of non-carbon elements no less than 5% by weight (including blowing agent weight); and (d) heat treating the layer of graphene material at a first heat treatment temperature from 80° C. to 3,200° C. at a desired heating rate sufficient to induce volatile gas molecules from the non-carbon elements or to activate the blowing agent for producing the graphene foam having a density from 0.01 to 1.7 g/cm3 or a specific surface area from 50 to 3,000 m2/g.

FIELD OF THE INVENTION

The present invention relates generally to the field of carbon/graphitefoams and, more particularly, to a new form of porous graphitic materialherein referred to as graphene foam and the process for producing same.

BACKGROUND OF THE INVENTION

Carbon is known to have five unique crystalline structures, includingdiamond, fullerene (0-D nano graphitic material), carbon nano-tube orcarbon nano-fiber (1-D nano graphitic material), graphene (2-D nanographitic material), and graphite (3-D graphitic material). The carbonnano-tube (CNT) refers to a tubular structure grown with a single wallor multi-wall. Carbon nano-tubes (CNTs) and carbon nano-fibers (CNFs)have a diameter on the order of a few nanometers to a few hundrednanometers. Their longitudinal, hollow structures impart uniquemechanical, electrical and chemical properties to the material. The CNTor CNF is a one-dimensional nano carbon or 1-D nano graphite material.

Bulk natural graphite is a 3-D graphitic material with each graphiteparticle being composed of multiple grains (a grain being a graphitesingle crystal or crystallite) with grain boundaries (amorphous ordefect zones) demarcating neighboring graphite single crystals. Eachgrain is composed of multiple graphene planes that are oriented parallelto one another. A graphene plane in a graphite crystallite is composedof carbon atoms occupying a two-dimensional, hexagonal lattice. In agiven grain or single crystal, the graphene planes are stacked andbonded via van der Waal forces in the crystallographic c-direction(perpendicular to the graphene plane or basal plane). Although all thegraphene planes in one grain are parallel to one another, typically thegraphene planes in one grain and the graphene planes in an adjacentgrain are inclined at different orientations. In other words, theorientations of the various grains in a graphite particle typicallydiffer from one grain to another.

The constituent graphene planes of a graphite crystallite in a naturalor artificial graphite particle can be exfoliated and extracted orisolated to obtain individual graphene sheets of carbon atoms providedthe inter-planar van der Waals forces can be overcome. An isolated,individual graphene sheet of carbon atoms is commonly referred to assingle-layer graphene. A stack of multiple graphene planes bondedthrough van der Waals forces in the thickness direction with aninter-graphene plane spacing of approximately 0.3354 nm is commonlyreferred to as a multi-layer graphene. A multi-layer graphene platelethas up to 300 layers of graphene planes (<100 nm in thickness), but moretypically up to 30 graphene planes (<10 nm in thickness), even moretypically up to 20 graphene planes (<7 nm in thickness), and mosttypically up to 10 graphene planes (commonly referred to as few-layergraphene in scientific community). Single-layer graphene and multi-layergraphene sheets are collectively called “nano graphene platelets”(NGPs). Graphene or graphene oxide sheets/platelets (collectively, NGPs)are a new class of carbon nano material (a 2-D nano carbon) that isdistinct from the 0-D fullerene, the 1-D CNT, and the 3-D graphite.

Our research group pioneered the development of graphene materials andrelated production processes as early as 2002: (1) B. Z. Jang and W. C.Huang, “Nano-scaled Graphene Plates,” U.S. Pat. No. 7,071,258 (Jul. 4,2006), application submitted on Oct. 21, 2002; (2) B. Z. Jang, et al.“Process for Producing Nano-scaled Graphene Plates,” U.S. patentapplication Ser. No. 10/858,814 (Jun. 3, 2004); and (3) B. Z. Jang, A.Zhamu, and J. Guo, “Process for Producing Nano-scaled Platelets andNanocomposites,” U.S. patent application Ser. No. 11/509,424 (Aug. 25,2006).

In one process, graphene materials are obtained by intercalating naturalgraphite particles with a strong acid and/or an oxidizing agent toobtain a graphite intercalation compound (GIC) or graphite oxide (GO),as illustrated in FIG. 1(A) (process flow chart) and FIG. 1(B)(schematic drawing). The presence of chemical species or functionalgroups in the interstitial spaces between graphene planes serves toincrease the inter-graphene spacing (d₀₀₂, as determined by X-raydiffraction), thereby significantly reducing the van der Waals forcesthat otherwise hold graphene planes together along the c-axis direction.The GIC or GO is most often produced by immersing natural graphitepowder (20 in FIG. 1(A) and 100 in FIG. 1(B)) in a mixture of sulfuricacid, nitric acid (an oxidizing agent), and another oxidizing agent(e.g. potassium permanganate or sodium perchlorate). The resulting GIC(22 or 102) is actually some type of graphite oxide (GO) particles if anoxidizing agent is present during the intercalation procedure. This GICor GO is then repeatedly washed and rinsed in water to remove excessacids, resulting in a graphite oxide suspension or dispersion, whichcontains discrete and visually discernible graphite oxide particlesdispersed in water. In order to produce graphene materials, one canfollow one of the two processing routes after this rinsing step, brieflydescribed below:

Route 1 involves removing water from the suspension to obtain“expandable graphite,” which is essentially a mass of dried GIC or driedgraphite oxide particles. Upon exposure of expandable graphite to atemperature in the range of typically 800-1,050° C. for approximately 30seconds to 2 minutes, the GIC undergoes a rapid volume expansion by afactor of 30-300 to form “graphite worms” (24 or 104), which are each acollection of exfoliated, but largely unseparated graphite flakes thatremain interconnected. A SEM image of graphite worms is presented inFIG. 2.

In Route 1A, these graphite worms (exfoliated graphite or “networks ofinterconnected/non-separated graphite flakes”) can be re-compressed toobtain flexible graphite sheets or foils (26 or 106) that typically havea thickness in the range of 0.1 mm (100 μm)-0.5 mm (500 μm).Alternatively, one may choose to use a low-intensity air mill orshearing machine to simply break up the graphite worms for the purposeof producing the so-called “expanded graphite flakes” (49 or 108) whichcontain mostly graphite flakes or platelets thicker than 100 nm (hence,not a nano material by definition).

In Route 1B, the exfoliated graphite is subjected to high-intensitymechanical shearing (e.g. using an ultrasonicator, high-shear mixer,high-intensity air jet mill, or high-energy ball mill) to form separatedsingle-layer and multi-layer graphene sheets (collectively called NGPs,33 or 112), as disclosed in our U.S. application Ser. No. 10/858,814.Single-layer graphene can be as thin as 0.34 nm, while multi-layergraphene can have a thickness up to 100 nm, but more typically less than10 nm (commonly referred to as few-layer graphene). Multiple graphenesheets or platelets may be made into a sheet of NGP paper (34) using apaper-making process.

Route 2 entails ultrasonicating the graphite oxide suspension for thepurpose of separating/isolating individual graphene oxide sheets fromgraphite oxide particles. This is based on the notion that theinter-graphene plane separation has been increased from 0.3354 nm innatural graphite to 0.6-1.1 nm in highly oxidized graphite oxide,significantly weakening the van der Waals forces that hold neighboringplanes together. Ultrasonic power can be sufficient to further separategraphene plane sheets to form separated, isolated, or discrete grapheneoxide (GO) sheets. These graphene oxide sheets can then be chemically orthermally reduced to obtain “reduced graphene oxides” (RGO) typicallyhaving an oxygen content of 0.001%-10% by weight, more typically0.01%-5% by weight, most typically and preferably less than 2% byweight.

For the purpose of defining the claims of the instant application, NGPsor graphene materials include discrete sheets/platelets of single-layerand multi-layer (typically less than 10 layers) pristine graphene,graphene oxide, reduced graphene oxide (RGO), graphene fluoride,graphene chloride, graphene bromide, graphene iodide, hydrogenatedgraphene, nitrogenated graphene, chemically functionalized graphene,doped graphene (e.g. doped by B or N). Pristine graphene has essentially0% oxygen. RGO typically has an oxygen content of 0.001%-5% by weight.Graphene oxide (including RGO) can have 0.001%-50% by weight of oxygen.Other than pristine graphene, all the graphene materials have 0.001%-50%by weight of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.).These materials are herein referred to as non-pristine graphenematerials.

Another process for producing graphene, in a thin film form (typically<2 nm in thickness), is the catalytic chemical vapor deposition process.This catalytic CVD involves catalytic decomposition of hydrocarbon gas(e.g. C₂H₄) on Ni or Cu surface to form single-layer or few-layergraphene. With Ni or Cu being the catalyst, carbon atoms obtained viadecomposition of hydrocarbon gas molecules at a temperature of800-1,000° C. are directly deposited onto Cu foil surface orprecipitated out to the surface of a Ni foil from a Ni—C solid solutionstate to form a sheet of single-layer or few-layer graphene (less than 5layers). The Ni- or Cu-catalyzed CVD process does not lend itself to thedeposition of more than 5 graphene planes (typically <2 nm) beyond whichthe underlying Ni or Cu layer can no longer provide any catalyticeffect. The CVD graphene films are extremely expensive.

Generally speaking, a foam or foamed material is composed of pores (orcells) and pore walls (a solid material). The pores can beinterconnected to form an open-cell foam. A graphene foam is composed ofpores and pore walls that contain a graphene material. There are threemajor methods of producing graphene foams:

The first method is the hydrothermal reduction of graphene oxidehydrogel that typically involves sealing graphene oxide (GO) aqueoussuspension in a high-pressure autoclave and heating the GO suspensionunder a high pressure (tens or hundreds of atm) at a temperaturetypically in the range of 180-300° C. for an extended period of time(typically 12-36 hours). A useful reference for this method is givenhere: Y. Xu, et al. “Self-Assembled Graphene Hydrogel via a One-StepHydrothermal Process,” ACS Nano 2010, 4, 4324-4330. There are severalmajor issues associated with this method: (a) The high pressurerequirement makes it an impractical method for industrial-scaleproduction. For one thing, this process cannot be conducted on acontinuous basis. (b) It is difficult, if not impossible, to exercisecontrol over the pore size and the porosity level of the resultingporous structure. (c) There is no flexibility in terms of varying theshape and size of the resulting reduced graphene oxide (RGO) material(e.g. it cannot be made into a film shape). (d) The method involves theuse of an ultra-low concentration of GO suspended in water (e.g. 2mg/mL=2 g/L=2 kg/kL). With the removal of non-carbon elements (up to50%), one can only produce less than 2 kg of graphene material (RGO) per1000-liter suspension. Furthermore, it is practically impossible tooperate a 1000-liter reactor that has to withstand the conditions of ahigh temperature and a high pressure. Clearly, this is not a scalableprocess for mass production of porous graphene structures.

The second method is based on a template-assisted catalytic CVD process,which involves CVD deposition of graphene on a sacrificial template(e.g. Ni foam). The graphene material conforms to the shape anddimensions of the Ni foam structure. The Ni foam is then etched awayusing an etching agent, leaving behind a monolith of graphene skeletonthat is essentially an open-cell foam. A useful reference for thismethod is given here: Zongping Chen, et al., “Three-dimensional flexibleand conductive interconnected graphene networks grown by chemical vapourdeposition,” Nature Materials, 10 (June 2011) 424-428. There are severalproblems associated with such a process: (a) the catalytic CVD isintrinsically a very slow, highly energy-intensive, and expensiveprocess; (b) the etching agent is typically a highly undesirablechemical and the resulting Ni-containing etching solution is a source ofpollution. It is very difficult and expensive to recover or recycle thedissolved Ni metal from the etchant solution. (c) It is challenging tomaintain the shape and dimensions of the graphene foam without damagingthe cell walls when the Ni foam is being etched away. The resultinggraphene foam is typically very brittle and fragile. (d) The transportof the CVD precursor gas (e.g. hydrocarbon) into the interior of a metalfoam can be difficult, resulting in a non-uniform structure, sincecertain spots inside the sacrificial metal foam may not be accessible tothe CVD precursor gas.

The third method of producing graphene foam also makes use of asacrificial material (e.g. colloidal polystyrene particles, PS) that iscoated with graphene oxide sheets using a self-assembly approach. Forinstance, Choi, et al. prepared chemically modified graphene (CMG) paperin two steps: fabrication of free-standing PS/CMG films by vacuumfiltration of a mixed aqueous colloidal suspension of CMG and PS (2.0 μmPS spheres), followed by removal of PS beads to generate 3D macro-pores.[B. G. Choi, et al., “3D Macroporous Graphene Frameworks forSupercapacitors with High Energy and Power Densities,” ACS Nano, 6(2012) 4020-4028.] Choi, et al. fabricated well-ordered free-standingPS/CMG paper by filtration, which began with separately preparing anegatively charged CMG colloidal and a positively charged PS suspension.A mixture of CMG colloidal and PS suspension was dispersed in solutionunder controlled pH (=2), where the two compounds had the same surfacecharges (zeta potential values of +13±2.4 mV for CMG and +68±5.6 mV forPS). When the pH was raised to 6, CMGs (zeta potential=−29±3.7 mV) andPS spheres (zeta potential=+51±2.5 mV) were assembled due to theelectrostatic interactions and hydrophobic characteristics between them,and these were subsequently integrated into PS/CMG composite paperthrough a filtering process. This method also has several shortcomings:(a) This method requires very tedious chemical treatments of bothgraphene oxide and PS particles. (b) The removal of PS by toluene alsoleads to weakened macro-porous structures. (c) Toluene is a highlyregulated chemical and must be treated with extreme caution. (d) Thepore sizes are typically excessively big (e.g. several μm), too big formany useful applications.

The above discussion clearly indicates that every prior art method orprocess for producing graphene foams has major deficiencies. Thus, it isan object of the present invention to provide a cost-effective processfor producing highly conductive, mechanically robust graphene foams inlarge quantities. This process does not involve the use of anenvironmentally unfriendly chemical. This process enables the flexibledesign and control of the porosity level and pore sizes.

It is another object of the present invention to provide a process forproducing graphene foams that exhibit a thermal conductivity, electricalconductivity, elastic modulus, and/or compressive strength that iscomparable to or greater than those of the graphite/carbon foams.

It is another object of the present invention to provide a process forproducing graphene foam in a thin film or sheet form (e.g. from 10 nm to300 μm) to facilitate its application as a current collector and/orelectrode material in a supercapacitor or battery. Such a thin filmexhibits a combination of exceptional thermal conductivity, electricalconductivity, mechanical strength, and elastic modulus unmatched by anygraphitic material of comparable specific gravity range.

Yet another object of the present invention is to provide (a) a pristinegraphene foam that contains essentially all carbon only and preferablyhave a meso-scaled pore size range (2-50 nm); and (b) non-pristinegraphene foams (graphene fluoride, graphene chloride, nitrogenatedgraphene, etc.) that contains at least 0.001% by weight (typically from0.01% to 5% by weight and most typically from 0.01% to 2%) of non-carbonelements.

SUMMARY OF THE INVENTION

The present invention provides a solid graphene foam composed ofmultiple pores and pore walls, wherein the pore walls contain a pristinegraphene material having essentially zero % of non-carbon elements, or anon-pristine graphene material having from 0.001% to 5% by weight ofnon-carbon elements wherein the non-pristine graphene is selected fromgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, or acombination thereof, and the solid graphene foam has a density from 0.01to 1.7 g/cm³, a specific surface area from 50 to 3,000 m²/g, a thermalconductivity of at least 200 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 2,000 S/cm per unit of specificgravity. In a preferred embodiment, the pore walls contain stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.36 nm as measured by X-ray diffraction.

In an embodiment, the pore walls contain a pristine graphene and thesolid graphene foam has a density from 0.5 to 1.7 g/cm³ or an averagepore size from 2 nm to 50 nm. In an embodiment, the pore walls contain anon-pristine graphene material selected from the group consisting ofgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, andcombinations thereof, and wherein the solid graphene foam contains acontent of non-carbon elements in the range of 0.01% to 2.0% by weight.In other words, the non-carbon elements can include an element selectedfrom oxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron In a specific embodiment, the pore walls contain graphene fluorideand the solid graphene foam contains a fluorine content from 0.01% to2.0% by weight. In another embodiment, the pore walls contain grapheneoxide and said solid graphene foam contains an oxygen content from 0.01%to 2.0% by weight. In an embodiment, the solid graphene foam has aspecific surface area from 200 to 2,000 m²/g or a density from 0.1 to1.5 g/cm³.

In a preferred embodiment, the solid graphene foam is made into acontinuous-length roll sheet form (a roll of a continuous foam sheet)having a thickness no greater than 200 μm and a length of at least 1meter long, preferably at least 2 meters, further preferably at least 10meters, and most preferably at least 100 meters. This sheet roll isproduced by a roll-to-roll process. There has been no prior art graphenefoam that is made into a sheet roll form. It has not been previouslyfound or suggested possible to have a roll-to-roll process for producinga continuous length of graphene foam, either pristine or non-pristine.

In a preferred embodiment, the graphene foam has an oxygen content ornon-carbon content less than 1% by weight, and the pore walls havestacked graphene planes having an inter-graphene spacing less than 0.35nm, a thermal conductivity of at least 250 W/mK per unit of specificgravity, and/or an electrical conductivity no less than 2,500 S/cm perunit of specific gravity.

In a further preferred embodiment, the graphene foam has an oxygencontent or non-carbon content less than 0.01% by weight and said porewalls contain stacked graphene planes having an inter-graphene spacingless than 0.34 nm, a thermal conductivity of at least 300 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,000 S/cm per unit of specific gravity.

In yet another preferred embodiment, the graphene foam has an oxygencontent or non-carbon content no greater than 0.01% by weight and saidpore walls contain stacked graphene planes having an inter-graphenespacing less than 0.336 nm, a mosaic spread value no greater than 0.7, athermal conductivity of at least 350 W/mK per unit of specific gravity,and/or an electrical conductivity no less than 3,500 S/cm per unit ofspecific gravity.

In still another preferred embodiment, the graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, a thermalconductivity greater than 400 W/mK per unit of specific gravity, and/oran electrical conductivity greater than 4,000 S/cm per unit of specificgravity.

In a preferred embodiment, the pore walls contain stacked grapheneplanes having an inter-graphene spacing less than 0.337 nm and a mosaicspread value less than 1.0. In a preferred embodiment, the graphene foamexhibits a degree of graphitization no less than 80% (preferably no lessthan 90%) and/or a mosaic spread value less than 0.4. In a preferredembodiment, the pore walls contain a 3D network of interconnectedgraphene planes.

In a preferred embodiment, the solid graphene foam contains meso-scaledpores having a pore size from 2 nm to 50 nm. The solid graphene foam canalso be made to contain micron-scaled pores (1-500 μm).

The presently invented solid graphene foam may be produced by a processcomprising:

(a) preparing a graphene dispersion having a graphene material dispersedin a liquid medium, wherein the graphene material is selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof and wherein the dispersion containsan optional blowing agent;(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a wet layer of graphenematerial, wherein the dispensing and depositing procedure includessubjecting the graphene dispersion to an orientation-inducing stress;(c) partially or completely removing the liquid medium from the wetlayer of graphene material to form a dried layer of graphene materialhaving a content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I,etc.) no less than 5% by weight; and(d) heat treating the dried layer of graphene material at a first heattreatment temperature from 100° C. to 3,200° C. at a desired heatingrate sufficient to induce volatile gas molecules from the non-carbonelements or to activate said blowing agent for producing the solidgraphene foam having a density from 0.01 to 1.7 g/cm³ (more typicallyfrom 0.1 to 1.5 g/cm³, and even more typically from 0.1 to 1.0 g/cm³,and most typically from 0.2 to 0.75 g/cm³), or a specific surface areafrom 50 to 3,000 m²/g (more typically from 200 to 2,000 m²/g, and mosttypically from 500 to 1,500 m²/g).

This optional blowing agent is not required if the graphene material hasa content of non-carbon elements (e.g. O, H, N, B, F, Cl, Br, I, etc.)no less than 5% by weight (preferably no less than 10%, furtherpreferably no less than 20%, even more preferably no less than 30% or40%, and most preferably up to 50%). The subsequent high temperaturetreatment serves to remove a majority of these non-carbon elements fromthe graphene material, generating volatile gas species that producepores or cells in the solid graphene material structure. In other words,quite surprisingly, these non-carbon elements play the role of a blowingagent. Hence, an externally added blowing agent is optional (notrequired). However, the use of a blowing agent can provide addedflexibility in regulating or adjusting the porosity level and pore sizesfor a desired application. The blowing agent is typically required ifthe non-carbon element content is less than 5%, such as pristinegraphene that is essentially all-carbon.

The blowing agent can be a physical blowing agent, a chemical blowingagent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.

The process may further include a step of heat-treating the solidgraphene foam at a second heat treatment temperature higher than thefirst heat treatment temperature for a length of time sufficient forobtaining a graphene foam wherein the pore walls contain stackedgraphene planes having an inter-plane spacing d₀₀₂ from 0.3354 nm to0.40 nm and a content of non-carbon elements less than 5% by weight(typically from 0.001% to 2%). When the resulting non-carbon elementcontent is from 0.1% to 2.0%, the inter-plane spacing d₀₀₂ is typicallyfrom 0.337 nm to 0.40 nm.

If the original graphene material in the dispersion contains anon-carbon element content higher than 5% by weight, the graphenematerial in the solid graphene foam (after the heat treatment) containsstructural defects that are induced during the step (d) of heattreating. The liquid medium can be simply water and/or an alcohol, whichis environmentally benign.

In a preferred embodiment, the process is a roll-to-roll process whereinsteps (b) and (c) include feeding the supporting substrate from a feederroller to a deposition zone, continuously or intermittently depositingthe graphene dispersion onto a surface of the supporting substrate toform the wet layer of graphene material thereon, drying the wet layer ofgraphene material to form the dried layer of graphene material, andcollecting the dried layer of graphene material deposited on thesupporting substrate on a collector roller. Such a roll-to-roll orreel-to-reel process is a truly industrial-scale, massive manufacturingprocess that can be automated.

In one embodiment, the first heat treatment temperature is from 100° C.to 1,500° C. In another embodiment, the second heat treatmenttemperature includes at least a temperature selected from (A) 300-1,500°C., (B) 1,500-2,100° C., and/or (C) 2,100-3,200° C. In a specificembodiment, the second heat treatment temperature includes a temperaturein the range of 300-1,500° C. for at least 1 hour and then a temperaturein the range of 1,500-3,200° C. for at least 1 hour.

There are several surprising results of conducting first and/or secondheat treatments to the dried graphene layer, and different heattreatment temperature ranges enable us to achieve different purposes,such as (a) removal of non-carbon elements from the graphene material(e.g. thermal reduction of fluorinated graphene to obtain graphene orreduced graphene fluoride, RGF)) which generate volatile gases toproduce pores or cells in a graphene material, (b) activation of thechemical or physical blowing agent to produce pores or cells, (c)chemical merging or linking of graphene sheets to significantly increasethe lateral dimension of graphene sheets in the foam walls (solidportion of the foam), (d) healing of defects created duringfluorination, oxidation, or nitrogenation of graphene planes in agraphite particle, and (e) re-organization and perfection of graphiticdomains or graphite crystals. These different purposes or functions areachieved to different extents within different temperature ranges. Thenon-carbon elements typically include an element selected from oxygen,fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, or boron. Quitesurprisingly, even under low-temperature foaming conditions,heat-treating induces chemical linking, merging, or chemical bondingbetween graphene sheets, often in an edge-to-edge manner (some inface-to-face manner).

In one embodiment, the sheet of solid graphene foam has a specificsurface area from 200 to 2,000 m²/g. In one embodiment, the sheet ofsolid graphene foam has a density from 0.1 to 1.5 g/cm³. In anembodiment, step (d) of heat treating the layer of graphene material ata first heat treatment temperature is conducted under a compressivestress. In another embodiment, the process comprises a compression stepto reduce a thickness, pore size, or porosity level of the sheet ofgraphene foam. In some applications, the graphene foam has a thicknessno greater than 200 μm.

In an embodiment, the graphene dispersion has at least 3% by weight ofgraphene oxide dispersed in the liquid medium to form a liquid crystalphase. In another embodiment, the graphene dispersion contains agraphene oxide dispersion prepared by immersing a graphitic material ina powder or fibrous form in an oxidizing liquid in a reaction vessel ata reaction temperature for a length of time sufficient to obtain thegraphene dispersion wherein the graphitic material is selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof andwherein the graphene oxide has an oxygen content no less than 5% byweight.

In an embodiment, the first heat treatment temperature contains atemperature in the range of 80° C.-300° C. and, as a result, thegraphene foam has an oxygen content or non-carbon element content lessthan 5%, and the pore walls have an inter-graphene spacing less than0.40 nm, a thermal conductivity of at least 150 W/mK (more typically atleast 200 W/mk) per unit of specific gravity, and/or an electricalconductivity no less than 2,000 S/cm per unit of specific gravity.

In a preferred embodiment, the first and/or second heat treatmenttemperature contains a temperature in the range of 300° C.-1,500° C.and, as a result, the graphene foam has an oxygen content or non-carboncontent less than 1%, and the pore walls have an inter-graphene spacingless than 0.35 nm, a thermal conductivity of at least 250 W/mK per unitof specific gravity, and/or an electrical conductivity no less than2,500 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature in the range of 1,500° C.-2,100° C., the graphene foam hasan oxygen content or non-carbon content less than 0.01% and pore wallshave an inter-graphene spacing less than 0.34 nm, a thermal conductivityof at least 300 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 3,000 S/cm per unit of specific gravity.

When the first and/or second heat treatment temperature contains atemperature greater than 2,100° C., the graphene foam has an oxygencontent or non-carbon content no greater than 0.001% and pore walls havean inter-graphene spacing less than 0.336 nm, a mosaic spread value nogreater than 0.7, a thermal conductivity of at least 350 W/mK per unitof specific gravity, and/or an electrical conductivity no less than3,500 S/cm per unit of specific gravity.

If the first and/or second heat treatment temperature contains atemperature no less than 2,500° C., the graphene foam has pore wallscontaining stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.4, and a thermalconductivity greater than 400 W/mK per unit of specific gravity, and/oran electrical conductivity greater than 4,000 S/cm per unit of specificgravity.

In one embodiment, the pore walls contain stacked graphene planes havingan inter-graphene spacing less than 0.337 nm and a mosaic spread valueless than 1.0. In another embodiment, the solid wall portion of thegraphene foam exhibits a degree of graphitization no less than 80%and/or a mosaic spread value less than 0.4. In yet another embodiment,the solid wall portion of the graphene foam exhibits a degree ofgraphitization no less than 90% and/or a mosaic spread value no greaterthan 0.4.

Typically, the pore walls contain a 3D network of interconnectedgraphene planes that are electron-conducting pathways. The cell wallscontain graphitic domains or graphite crystals having a lateraldimension (L_(a), length or width) no less than 20 nm, more typicallyand preferably no less than 40 nm, still more typically and preferablyno less than 100 nm, still more typically and preferably no less than500 nm, often greater than 1 μm, and sometimes greater than 10 μm. Thegraphitic domains typically have a thickness from 1 nm to 200 nm, moretypically from 1 nm to 100 nm, further more typically from 1 nm to 40nm, and most typically from 1 nm to 30 nm.

Preferably, the solid graphene foam contains meso-scaled pores having apore size from 2 nm to 50 nm (preferably 2 nm to 25 nm) It may be notedthat it has not been possible to use Ni-catalyzed CVD to producegraphene foams having a pore size range of 2-50 nm. This is due to thenotion that it has not been proven possible to prepare Ni foam templateshaving such a pore size range and not possible for the hydrocarbon gas(precursor molecules) to readily enter Ni foam pores of these sizes.These Ni foam pores must also be interconnected. Additionally, thesacrificial plastic colloidal particle approaches have resulted inmacro-pores that are in the size range of microns to millimeters.

In a preferred embodiment, the present invention provides a roll-to-rollprocess for producing a solid graphene foam composed of multiple poresand pore walls The process comprises: (a) preparing a graphenedispersion having a graphene material dispersed in a liquid medium,wherein the dispersion optionally contains a blowing agent; (b)continuously or intermittently dispensing and depositing the graphenedispersion onto a surface of a supporting substrate to form a wet layerof graphene material, wherein the supporting substrate is a continuousthin film supplied from a feeder roller and collected on a collectorroller; (c) partially or completely removing the liquid medium from thewet layer of graphene material to form a dried layer of graphene; and(d) heat treating the dried layer of graphene material at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to activate the blowing agent for producing said solidgraphene foam having a density from 0.01 to 1.7 g/cm³ or a specificsurface area from 50 to 3,000 m²/g.

The orientation-inducing stress may be a shear stress. As an example,the shear stress can be encountered in a situation as simple as a“doctor's blade” that guides the spreading of graphene dispersion over aplastic or glass surface during a manual casting process. As anotherexample, an effective orientation-inducing stress is created in anautomated roll-to-roll coating process in which a “knife-on-roll”configuration dispenses the graphene dispersion over a moving solidsubstrate, such as a plastic film. The relative motion between thismoving film and the coating knife acts to effect orientation of graphenesheets along the shear stress direction.

This orientation-inducing stress is a critically important step in theproduction of the presently invented graphene foams due to thesurprising observation that the shear stress enables the graphene sheetsto align along a particular direction (e.g. X-direction orlength-direction) to produce preferred orientations and facilitatecontacts between graphene sheets along foam walls. Further surprisingly,these preferred orientations and improved graphene-to-graphene contactsfacilitate chemical merging or linking between graphene sheets duringthe subsequent heat treatment of the dried graphene layer. Suchpreferred orientations and improved contacts are essential to theeventual attainment of exceptionally high thermal conductivity,electrical conductivity, elastic modulus, and mechanical strength of theresulting graphene foam. In general, these great properties could not beobtained without such a shear stress-induced orientation control.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 (A) A flow chart illustrating various prior art processes ofproducing exfoliated graphite products (flexible graphite foils andexpanded graphite flakes), along with a process for producing pristinegraphene foam 40 a or graphene oxide foams 40 b; (B) Schematic drawingillustrating the processes for producing conventional paper, mat, film,and membrane of simply aggregated graphite or NGP flakes/platelets. Allprocesses begin with intercalation and/or oxidation treatment ofgraphitic materials (e.g. natural graphite particles).

FIG. 2 A SEM image of a graphite worm sample after thermal exfoliationof graphite intercalation compounds (GICs) or graphite oxide powders.

FIG. 3 A possible mechanism of chemical linking between graphene oxidesheets that effectively increases the graphene sheet lateral dimensions.

FIG. 4 (A) Thermal conductivity values vs. specific gravity of the GOsuspension-derived foam produced by the presently invented process,meso-phase pitch-derived graphite foam, and Ni foam-template assistedCVD graphene foam; (B) Thermal conductivity values of the GOsuspension-derived foam, sacrificial plastic bead-templated GO foam, andthe hydrothermally reduced GO graphene foam; and (C) electricalconductivity data for the GO suspension-derived foam produced by thepresently invented process and the hydrothermally reduced GO graphenefoam.

FIG. 5 (A) Thermal conductivity values (vs. specific gravity values upto 1.02 g/cm³) of the GO suspension-derived foam, meso-phasepitch-derived graphite foam, and Ni foam-template assisted CVD graphenefoam; (B) Thermal conductivity values of the GO suspension-derived foam,sacrificial plastic bead-templated GO foam, and hydrothermally reducedGO graphene foam (vs. specific gravity values up to 1.02 g/cm³).

FIG. 6 Thermal conductivity values of graphene foam samples derived fromGO and GF (graphene fluoride) as a function of the specific gravity.

FIG. 7 Thermal conductivity values of graphene foam samples derived fromGO and pristine graphene as a function of the final (maximum) heattreatment temperature.

FIG. 8 (A) Inter-graphene plane spacing in graphene foam walls asmeasured by X-ray diffraction; (B) the oxygen content in the GOsuspension-derived graphene foam.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides a sheet of solid graphene foam composedof multiple pores and pore walls and a process for producing same. Thepores in the graphene foam are formed slightly before, during, or aftersheets of a graphene material are (1) chemically linked/merged together(edge-to-edge and/or face-to-face) typically at a temperature from 100to 1,500° C. and/or (2) re-organized into larger graphite crystals ordomains (herein referred to as re-graphitization) along the pore wallsat a high temperature (typically >2,100° C. and more typically >2,500°C.). The process comprises:

(a) preparing a graphene dispersion having a graphene material dispersedin a liquid medium, wherein the graphene material is selected frompristine graphene, graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof and wherein the dispersion containsan optional blowing agent with a blowing agent-to-graphene materialweight ratio from 0/1.0 to 1.0/1.0 (this blowing agent is normallyrequired if the graphene material is pristine graphene, typically havinga blowing agent-to-pristine graphene weight ratio from 0.01/1.0 to1.0/1.0);

(b) dispensing and depositing the graphene dispersion onto a surface ofa supporting substrate (e.g. plastic film, rubber sheet, metal foil,glass sheet, paper sheet, etc.) to form a first wet layer of graphenematerial, wherein the dispensing and depositing procedure (e.g. coatingor casting) includes subjecting the graphene dispersion to anorientation-inducing stress;

(c) partially or completely removing the liquid medium from the firstwet layer of graphene material to form a first dried layer of graphenematerial having a content of non-carbon elements (e.g. O, H, N, B, F,Cl, Br, I, etc.) no less than 5% by weight (this non-carbon content,when being removed via heat-induced decomposition, produces volatilegases that act as a foaming agent or blowing agent); and

(d) heat treating the first layer of graphene material at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to induce volatile gas molecules from the non-carbonelements or to activate said blowing agent for producing the solidgraphene foam. The graphene foam typically has a density from 0.01 to1.7 g/cm³ (more typically from 0.1 to 1.5 g/cm³, and even more typicallyfrom 0.1 to 1.0 g/cm³, and most typically from 0.2 to 0.75 g/cm³), or aspecific surface area from 50 to 3,000 m²/g (more typically from 200 to2,000 m²/g, and most typically from 500 to 1,500 m²/g).

A blowing agent or foaming agent is a substance which is capable ofproducing a cellular or foamed structure via a foaming process in avariety of materials that undergo hardening or phase transition, such aspolymers (plastics and rubbers), glass, and metals. They are typicallyapplied when the material being foamed is in a liquid state. It has notbeen previously known that a blowing agent can be used to create afoamed material while in a solid state. More significantly, it has notbeen taught or hinted that an aggregate of sheets of a graphene materialcan be converted into a graphene foam via a blowing agent. The cellularstructure in a matrix is typically created for the purpose of reducingdensity, increasing thermal resistance and acoustic insulation, whileincreasing the thickness and relative stiffness of the original polymer.

Blowing agents or related foaming mechanisms to create pores or cells(bubbles) in a matrix for producing a foamed or cellular material, canbe classified into the following groups:

-   -   (a) Physical blowing agents: e.g. hydrocarbons (e.g. pentane,        isopentane, cyclopentane), chlorofluorocarbons (CFCs),        hydrochlorofluorocarbons (HCFCs), and liquid CO₂. The        bubble/foam-producing process is endothermic, i.e. it needs heat        (e.g. from a melt process or the chemical exotherm due to        cross-linking), to volatize a liquid blowing agent.    -   (b) Chemical blowing agents: e.g. isocyanate, azo-, hydrazine        and other nitrogen-based materials (for thermoplastic and        elastomeric foams), sodium bicarbonate (e.g. baking soda, used        in thermoplastic foams). Here gaseous products and other        by-products are formed by a chemical reaction, promoted by        process or a reacting polymer's exothermic heat. Since the        blowing reaction involves forming low molecular weight compounds        that act as the blowing gas, additional exothermic heat is also        released. Powdered titanium hydride is used as a foaming agent        in the production of metal foams, as it decomposes to form        titanium and hydrogen gas at elevated temperatures.        Zirconium (II) hydride is used for the same purpose. Once formed        the low molecular weight compounds will never revert to the        original blowing agent(s), i.e. the reaction is irreversible.    -   (c) Mixed physical/chemical blowing agents: e.g. used to produce        flexible polyurethane (PU) foams with very low densities. Both        the chemical and physical blowing can be used in tandem to        balance each other out with respect to thermal energy        released/absorbed; hence, minimizing temperature rise. For        instance, isocyanate and water (which react to form CO₂) are        used in combination with liquid CO₂ (which boils to give gaseous        form) in the production of very low density flexible PU foams        for mattresses.    -   (d) Mechanically injected agents: Mechanically made foams        involve methods of introducing bubbles into liquid polymerizable        matrices (e.g. an unvulcanized elastomer in the form of a liquid        latex). Methods include whisking-in air or other gases or low        boiling volatile liquids in low viscosity lattices, or the        injection of a gas into an extruder barrel or a die, or into        injection molding barrels or nozzles and allowing the shear/mix        action of the screw to disperse the gas uniformly to form very        fine bubbles or a solution of gas in the melt. When the melt is        molded or extruded and the part is at atmospheric pressure, the        gas comes out of solution expanding the polymer melt immediately        before solidification.    -   (e) Soluble and leachable agents: Soluble fillers, e.g. solid        sodium chloride crystals mixed into a liquid urethane system,        which is then shaped into a solid polymer part, the sodium        chloride is later washed out by immersing the solid molded part        in water for some time, to leave small inter-connected holes in        relatively high density polymer products.    -   (f) We have found that the above five mechanisms can all be used        to create pores in the graphene materials while they are in a        solid state. Another mechanism of producing pores in a graphene        material is through the generation and vaporization of volatile        gases by removing those non-carbon elements in a        high-temperature environment. This is a unique self-foaming        process that has never been previously taught or suggested.

In a preferred embodiment, the graphene material in the dispersion isselected from pristine graphene, graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof. The starting graphitic material forproducing any one of the above graphene materials may be selected fromnatural graphite, artificial graphite, meso-phase carbon, meso-phasepitch, meso-carbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nano-fiber, carbon nano-tube, or a combination thereof.

For instance, as discussed in the Background section, the graphene oxide(GO) may be obtained by immersing powders or filaments of a startinggraphitic material (e.g. natural graphite powder) in an oxidizing liquidmedium (e.g. a mixture of sulfuric acid, nitric acid, and potassiumpermanganate) in a reaction vessel at a desired temperature for a periodof time (typically from 0.5 to 96 hours, depending upon the nature ofthe starting material and the type of oxidizing agent used). Theresulting graphite oxide particles may then be subjected to thermalexfoliation or ultrasonic wave-induced exfoliation to produce GO sheets.

Pristine graphene may be produced by direct ultrasonication (also knownas liquid phase production) or supercritical fluid exfoliation ofgraphite particles. These processes are well-known in the art. Multiplepristine graphene sheets may be dispersed in water or other liquidmedium with the assistance of a surfactant to form a suspension. Achemical blowing agent may then be dispersed into the dispersion (38 inFIG. 1(A)). This suspension is then cast or coated onto the surface of asolid substrate (e.g. glass sheet or Al foil). When heated to a desiredtemperature, the chemical blowing agent is activated or decomposed togenerate volatile gases (e.g. N₂ or CO₂), which act to form bubbles orpores in an otherwise mass of solid graphene sheets, forming a pristinegraphene foam 40 a.

Fluorinated graphene or graphene fluoride is herein used as an exampleof the halogenated graphene material group. There are two differentapproaches that have been followed to produce fluorinated graphene: (1)fluorination of pre-synthesized graphene: This approach entails treatinggraphene prepared by mechanical exfoliation or by CVD growth withfluorinating agent such as XeF₂, or F-based plasmas; (2) Exfoliation ofmultilayered graphite fluorides: Both mechanical exfoliation and liquidphase exfoliation of graphite fluoride can be readily accomplished [F.Karlicky, et al. “Halogenated Graphenes: Rapidly Growing Family ofGraphene Derivatives” ACS Nano, 2013, 7 (8), pp 6434-6464].

Interaction of F₂ with graphite at high temperature leads to covalentgraphite fluorides (CF)_(n) or (C₂F)_(n), while at low temperaturesgraphite intercalation compounds (GIC) C_(x)F (2≤x≤24) form. In (CF)_(n)carbon atoms are sp3-hybridized and thus the fluorocarbon layers arecorrugated consisting of trans-linked cyclohexane chairs. In (C₂F)_(n)only half of the C atoms are fluorinated and every pair of the adjacentcarbon sheets are linked together by covalent C—C bonds. Systematicstudies on the fluorination reaction showed that the resulting F/C ratiois largely dependent on the fluorination temperature, the partialpressure of the fluorine in the fluorinating gas, and physicalcharacteristics of the graphite precursor, including the degree ofgraphitization, particle size, and specific surface area. In addition tofluorine (F₂), other fluorinating agents may be used, although most ofthe available literature involves fluorination with F₂ gas, sometimes inpresence of fluorides.

For exfoliating a layered precursor material to the state of individuallayers or few-layers, it is necessary to overcome the attractive forcesbetween adjacent layers and to further stabilize the layers. This may beachieved by either covalent modification of the graphene surface byfunctional groups or by non-covalent modification using specificsolvents, surfactants, polymers, or donor-acceptor aromatic molecules.The process of liquid phase exfoliation includes ultra-sonic treatmentof a graphite fluoride in a liquid medium.

The nitrogenation of graphene can be conducted by exposing a graphenematerial, such as graphene oxide, to ammonia at high temperatures(200-400° C.). Nitrogenated graphene could also be formed at lowertemperatures by a hydrothermal method; e.g. by sealing GO and ammonia inan autoclave and then increased the temperature to 150-250° C. Othermethods to synthesize nitrogen doped graphene include nitrogen plasmatreatment on graphene, arc-discharge between graphite electrodes in thepresence of ammonia, ammonolysis of graphene oxide under CVD conditions,and hydrothermal treatment of graphene oxide and urea at differenttemperatures.

The pore walls (cell walls) in the presently invented graphene foamcontain chemically bonded and merged graphene planes. These planararomatic molecules or graphene planes (hexagonal structured carbonatoms) are well interconnected physically and chemically. The lateraldimensions (length or width) of these planes are huge (from 20 nm to >10μm), typically several times or even orders of magnitude larger than themaximum crystallite dimension (or maximum constituent graphene planedimension) of the starting graphite particles. The graphene sheets orplanes are essentially interconnected to form electron-conductingpathways with low resistance. This is a unique and new class of materialthat has not been previously discovered, developed, or suggested topossibly exist.

In order to illustrate how the presently invented process works toproduce a graphene foam, we herein make use of graphene oxide (GO) andgraphene fluoride (GF) as two examples. These should not be construed aslimiting the scope of our claims. In each case, the first step involvespreparation of a graphene dispersion (e.g. GO+water or GF+organicsolvent, DMF) containing an optional blowing agent. If the graphenematerial is pristine graphene containing no non-carbon elements, ablowing agent is required.

In step (b), the GF or GO suspension (21 in FIG. 1(A)) is formed into awet GF or GO layer 35 on a solid substrate surface (e.g. PET film orglass) preferably under the influence of a shear stress. One example ofsuch a shearing procedure is casting or coating a thin film of GF or GOsuspension using a coating machine. This procedure is similar to a layerof varnish, paint, coating, or ink being coated onto a solid substrate.The roller, “doctor's blade”, or wiper creates a shear stress when thefilm is shaped, or when there is a relative motion between theroller/blade/wiper and the supporting substrate. Quite unexpectedly andsignificantly, such a shearing action enables the planar GF or GO sheetsto well align along, for instance, a shearing direction. Furthersurprisingly, such a molecular alignment state or preferred orientationis not disrupted when the liquid components in the GF or GO suspensionare subsequently removed to form a well-packed layer of highly alignedGF or GO sheets that are at least partially dried. The dried GF or GOmass 37 a has a high birefringence coefficient between an in-planedirection and the normal-to-plane direction.

In an embodiment, this GF or GO layer is then subjected to a heattreatment to activate the blowing agent and/or the thermally-inducedreactions that remove the non-carbon elements (e.g. F, O, etc.) from thegraphene sheets to generate volatile gases as by-products. Thesevolatile gases generate pores or bubbles inside the solid graphenematerial, pushing solid graphene sheets into a wall structure, forming agraphene oxide foam 40 b. If no blowing agent is added, the non-carbonelements in the graphene material preferably occupy at least 10% byweight of the graphene material (preferably at least 20%, and furtherpreferably at least 30%). The first (initial) heat treatment temperatureis typically greater than 80° C., preferably greater than 100° C., morepreferably greater than 300° C., further more preferably greater than500° C. and can be as high as 1,500° C. The blowing agent is typicallyactivated at a temperature from 80° C. to 300° C., but can be higher.The foaming procedure (formation of pores, cells, or bubbles) istypically completed within the temperature range of 80-1,500° C. Quitesurprisingly, the chemical linking or merging between graphene planes(GO or GF planes) in an edge-to-edge and face-to-face manner can occurat a relatively low heat treatment temperature (e.g. as low as from 150to 300° C.).

The foamed graphene material may be subjected to a further heattreatment that involves at least a second temperature that issignificantly higher than the first heat treatment temperature.

A properly programmed heat treatment procedure can involve just a singleheat treatment temperature (e.g. a first heat treatment temperatureonly), at least two heat treatment temperatures (first temperature for aperiod of time and then raised to a second temperature and maintained atthis second temperature for another period of time), or any othercombination of heat treatment temperatures (HTT) that involve an initialtreatment temperature (first temperature) and a final HTT (second),higher than the first. The highest or final HTT that the dried graphenelayer experiences may be divided into four distinct HTT regimes:

-   Regime 1 (80° C. to 300° C.): In this temperature range (the thermal    reduction regime and also the activation regime for a blowing agent,    if present), a GO or GF layer primarily undergoes thermally-induced    reduction reactions, leading to a reduction of oxygen content or    fluorine content from typically 20-50% (of O in GO) or 10-25% (of F    in GF) to approximately 5-6%. This treatment results in a reduction    of inter-graphene spacing in foam walls from approximately 0.6-1.2    nm (as dried) down to approximately 0.4 nm, and an increase in    thermal conductivity to 200 W/mK per unit specific gravity and/or    electrical conductivity to 2,000 S/cm per unit of specific gravity.    (Since one can vary the level of porosity and, hence, specific    gravity of a graphene foam material and, given the same graphene    material, both the thermal conductivity and electric conductivity    values vary with the specific gravity, these property values must be    divided by the specific gravity to facilitate a fair comparison.)    Even with such a low temperature range, some chemical linking    between graphene sheets occurs. The inter-GO or inter-GF planar    spacing remains relatively large (0.4 nm or larger). Many O- or    F-containing functional groups survive.-   Regime 2 (300° C.-1,500° C.): In this chemical linking regime,    extensive chemical combination, polymerization, and cross-linking    between adjacent GO or GF sheets occur. The oxygen or fluorine    content is reduced to typically <1.0% (e.g. 0.7%) after chemical    linking, resulting in a reduction of inter-graphene spacing to    approximately 0.345 nm. This implies that some initial    re-graphitization has already begun at such a low temperature, in    stark contrast to conventional graphitizable materials (such as    carbonized polyimide film) that typically require a temperature as    high as 2,500° C. to initiate graphitization. This is another    distinct feature of the presently invented graphene foam and its    production processes. These chemical linking reactions result in an    increase in thermal conductivity to 250 W/mK per unit of specific    gravity, and/or electrical conductivity to 2,500-4,000 S/cm per unit    of specific gravity.-   Regime 3 (1,500-2,500° C.): In this ordering and re-graphitization    regime, extensive graphitization or graphene plane merging occurs,    leading to significantly improved degree of structural ordering in    the foam walls. As a result, the oxygen or fluorine content is    reduced to typically 0.01% and the inter-graphene spacing to    approximately 0.337 nm (achieving degree of graphitization from 1%    to approximately 80%, depending upon the actual HTT and length of    time). The improved degree of ordering is also reflected by an    increase in thermal conductivity to >350 W/mK per unit of specific    gravity, and/or electrical conductivity to >3,500 S/cm per unit of    specific gravity.-   Regime 4 (higher than 2,500° C.): In this re-crystallization and    perfection regime, extensive movement and elimination of grain    boundaries and other defects occur, resulting in the formation of    nearly perfect single crystals or poly-crystalline graphene crystals    with huge grains in the foam walls, which can be orders of magnitude    larger than the original grain sizes of the starting graphite    particles for the production of GO or GF. The oxygen or fluorine    content is essentially eliminated, typically 0%-0.001%. The    inter-graphene spacing is reduced to down to approximately 0.3354 nm    (degree of graphitization from 80% to nearly 100%), corresponding to    that of a perfect graphite single crystal. The foamed structure thus    obtained exhibits a thermal conductivity of >400 W/mK per unit of    specific gravity, and electrical conductivity of >4,000 S/cm per    unit of specific gravity.

The presently invented graphene foam structure can be obtained byheat-treating the dried GO or GF layer with a temperature program thatcovers at least the first regime (typically requiring 1-4 hours in thistemperature range if the temperature never exceeds 500° C.), morecommonly covers the first two regimes (1-2 hours preferred), still morecommonly the first three regimes (preferably 0.5-2.0 hours in Regime 3),and can cover all the 4 regimes (including Regime 4 for 0.2 to 1 hour,may be implemented to achieve the highest conductivity).

If the graphene material is selected from the group of non-pristinegraphene materials consisting of graphene oxide, reduced graphene oxide,graphene fluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, or a combination thereof, and wherein the maximum heattreatment temperature (e.g. both the first and second heat treatmenttemperatures) is (are) less than 2,500° C., then the resulting solidgraphene foam typically contains a content of non-carbon elements in therange of 0.01% to 2.0% by weight (non-pristine graphene foam).

X-ray diffraction patterns were obtained with an X-ray diffractometerequipped with CuKcv radiation. The shift and broadening of diffractionpeaks were calibrated using a silicon powder standard. The degree ofgraphitization, g, was calculated from the X-ray pattern using theMering's Eq, d₀₀₂=0.3354 g+0.344 (1−g), where d₀₀₂ is the interlayerspacing of graphite or graphene crystal in nm. This equation is validonly when d₀₀₂ is equal or less than approximately 0.3440 nm. Thegraphene foam walls having a d₀₀₂ higher than 0.3440 nm reflects thepresence of oxygen- or fluorine-containing functional groups (such as—F, —OH, >O, and —COOH on graphene molecular plane surfaces or edges)that act as a spacer to increase the inter-graphene spacing.

Another structural index that can be used to characterize the degree ofordering of the stacked and bonded graphene planes in the foam walls ofgraphene and conventional graphite crystals is the “mosaic spread,”which is expressed by the full width at half maximum of a rocking curve(X-ray diffraction intensity) of the (002) or (004) reflection. Thisdegree of ordering characterizes the graphite or graphene crystal size(or grain size), amounts of grain boundaries and other defects, and thedegree of preferred grain orientation. A nearly perfect single crystalof graphite is characterized by having a mosaic spread value of 0.2-0.4.Most of our graphene walls have a mosaic spread value in this range of0.2-0.4 (if produced with a heat treatment temperature (HTT) no lessthan 2,500° C.). However, some values are in the range of 0.4-0.7 if theHTT is between 1,500 and 2,500° C., and in the range of 0.7-1.0 if theHTT is between 300 and 1,500° C.

Illustrated in FIG. 3 is a plausible chemical linking mechanism whereonly 2 aligned GO molecules are shown as an example, although a largenumber of GO molecules can be chemically linked together to form a foamwall. Further, chemical linking could also occur face-to-face, not justedge-to-edge for GO, GF, and chemically functionalized graphene sheets.These linking and merging reactions proceed in such a manner that themolecules are chemically merged, linked, and integrated into one singleentity. The graphene sheets (GO or GF sheets) completely lose their ownoriginal identity and they no longer are discretesheets/platelets/flakes. The resulting product is not a simple aggregateof individual graphene sheets, but a single entity that is essentially anetwork of interconnected giant molecules with an essentially infinitemolecular weight. This may also be described as a graphene poly-crystal(with several grains, but typically no discernible, well-defined grainboundaries). All the constituent graphene planes are very large inlateral dimensions (length and width) and, if the HTT is sufficientlyhigh (e.g. >1,500° C. or much higher), these graphene planes areessentially bonded together with one another.

In-depth studies using a combination of SEM, TEM, selected areadiffraction, X-ray diffraction, AFM, Raman spectroscopy, and FTIRindicate that the graphene foam walls are composed of several hugegraphene planes (with length/width typically >>20 nm, moretypically >>100 nm, often >>1 μm, and, in many cases, >>10 μm, oreven >>100 μm). These giant graphene planes are stacked and bonded alongthe thickness direction (crystallographic c-axis direction) oftenthrough not just the van der Waals forces (as in conventional graphitecrystallites), but also covalent bonds, if the final heat treatmenttemperature is lower than 2,500° C. In these cases, wishing not to belimited by theory, but Raman and FTIR spectroscopy studies appear toindicate the co-existence of sp² (dominating) and sp³ (weak butexisting) electronic configurations, not just the conventional sp² ingraphite.

-   (1) This graphene foam wall is not made by gluing or bonding    discrete flakes/platelets together with a resin binder, linker, or    adhesive. Instead, GO sheets (molecules) from the GO dispersion or    the GF sheets from the GF dispersion are merged through joining or    forming of covalent bonds with one another, into an integrated    graphene entity, without using any externally added linker or binder    molecules or polymers.-   (2) This graphene foam wall is typically a poly-crystal composed of    large grains having incomplete grain boundaries. This entity is    derived from a GO or GF suspension, which is in turn obtained from    natural graphite or artificial graphite particles originally having    multiple graphite crystallites. Prior to being chemically oxidized    or fluorinated, these starting graphite crystallites have an initial    length (L_(a) in the crystallographic α-axis direction), initial    width (L_(b) in the b-axis direction), and thickness (L_(c) in the    c-axis direction). Upon oxidation or fluorination, these initially    discrete graphite particles are chemically transformed into highly    aromatic graphene oxide or graphene fluoride molecules having a    significant concentration of edge- or surface-borne functional    groups (e.g. —F, —OH, —COOH, etc.). These aromatic GO or GF    molecules in the suspension have lost their original identity of    being part of a graphite particle or flake. Upon removal of the    liquid component from the suspension, the resulting GO or GF    molecules form an essentially amorphous structure. Upon heat    treatments, these GO or GF molecules are chemically merged and    linked into a unitary or monolithic graphene entity that constitutes    the foam wall. This foam wall is highly ordered.

The resulting unitary graphene entity in the foam wall typically has alength or width significantly greater than the L_(a) and L_(b) of theoriginal crystallites. The length/width of this graphene foam wallentity is significantly greater than the L_(a) and L_(b) of the originalcrystallites. Even the individual grains in a poly-crystalline graphenewall structure have a length or width significantly greater than theL_(a) and L_(b) of the original crystallites.

-   (3) Due to these unique chemical composition (including oxygen or    fluorine content), morphology, crystal structure (including    inter-graphene spacing), and structural features (e.g. high degree    of orientations, few defects, incomplete grain boundaries, chemical    bonding and no gap between graphene sheets, and substantially no    interruptions in graphene planes), the GO- or GF-derived graphene    foam has a unique combination of outstanding thermal conductivity,    electrical conductivity, mechanical strength, and stiffness (elastic    modulus).

The aforementioned features are further described and explained indetail as follows: As illustrated in FIG. 1(B), a graphite particle(e.g. 100) is typically composed of multiple graphite crystallites orgrains. A graphite crystallite is made up of layer planes of hexagonalnetworks of carbon atoms. These layer planes of hexagonally arrangedcarbon atoms are substantially flat and are oriented or ordered so as tobe substantially parallel and equidistant to one another in a particularcrystallite. These layers of hexagonal-structured carbon atoms, commonlyreferred to as graphene layers or basal planes, are weakly bondedtogether in their thickness direction (crystallographic c-axisdirection) by weak van der Waals forces and groups of these graphenelayers are arranged in crystallites. The graphite crystallite structureis usually characterized in terms of two axes or directions: the c-axisdirection and the a-axis (or b-axis) direction. The c-axis is thedirection perpendicular to the basal planes. The a- or b-axes are thedirections parallel to the basal planes (perpendicular to the c-axisdirection).

A highly ordered graphite particle can consist of crystallites of aconsiderable size, having a length of L_(a) along the crystallographica-axis direction, a width of L_(b) along the crystallographic b-axisdirection, and a thickness L_(c) along the crystallographic c-axisdirection. The constituent graphene planes of a crystallite are highlyaligned or oriented with respect to each other and, hence, theseanisotropic structures give rise to many properties that are highlydirectional. For instance, the thermal and electrical conductivity of acrystallite are of great magnitude along the plane directions (a- orb-axis directions), but relatively low in the perpendicular direction(c-axis). As illustrated in the upper-left portion of FIG. 1(B),different crystallites in a graphite particle are typically oriented indifferent directions and, hence, a particular property of amulti-crystallite graphite particle is the directional average value ofall the constituent crystallites.

Due to the weak van der Waals forces holding the parallel graphenelayers, natural graphite can be treated so that the spacing between thegraphene layers can be appreciably opened up so as to provide a markedexpansion in the c-axis direction, and thus form an expanded graphitestructure in which the laminar character of the carbon layers issubstantially retained. The process for manufacturing flexible graphiteis well-known in the art. In general, flakes of natural graphite (e.g.100 in FIG. 1(B)) are intercalated in an acid solution to producegraphite intercalation compounds (GICs, 102). The GICs are washed,dried, and then exfoliated by exposure to a high temperature for a shortperiod of time. This causes the flakes to expand or exfoliate in thec-axis direction of the graphite up to 80-300 times of their originaldimensions. The exfoliated graphite flakes are vermiform in appearanceand, hence, are commonly referred to as worms 104. These worms ofgraphite flakes which have been greatly expanded can be formed withoutthe use of a binder into cohesive or integrated sheets of expandedgraphite, e.g. webs, papers, strips, tapes, foils, mats or the like(typically referred to as “flexible graphite” 106) having a typicaldensity of about 0.04-2.0 g/cm³ for most applications.

The upper left portion of FIG. 1(A) shows a flow chart that illustratesthe prior art processes used to fabricate flexible graphite foils. Theprocesses typically begin with intercalating graphite particles 20(e.g., natural graphite or synthetic graphite) with an intercalant(typically a strong acid or acid mixture) to obtain a graphiteintercalation compound 22 (GIC). After rinsing in water to remove excessacid, the GIC becomes “expandable graphite.” The GIC or expandablegraphite is then exposed to a high temperature environment (e.g., in atube furnace preset at a temperature in the range of 800-1,050° C.) fora short duration of time (typically from 15 seconds to 2 minutes). Thisthermal treatment allows the graphite to expand in its c-axis directionby a factor of 30 to several hundreds to obtain a worm-like vermicularstructure 24 (graphite worm), which contains exfoliated, butun-separated graphite flakes with large pores interposed between theseinterconnected flakes. An example of graphite worms is presented in FIG.2.

In one prior art process, the exfoliated graphite (or mass of graphiteworms) is re-compressed by using a calendaring or roll-pressingtechnique to obtain flexible graphite foils (26 in FIG. 1(A) or 106 inFIG. 1(B)), which are typically 100-300 μm thick. In another prior artprocess, the exfoliated graphite worm 24 may be impregnated with a resinand then compressed and cured to form a flexible graphite composite,which is normally of low strength as well. In addition, upon resinimpregnation, the electrical and thermal conductivity of the graphiteworms could be reduced by two orders of magnitude.

Alternatively, the exfoliated graphite may be subjected tohigh-intensity mechanical shearing/separation treatments using ahigh-intensity air jet mill, high-intensity ball mill, or ultrasonicdevice to produce separated nano graphene platelets 33 (NGPs) with allthe graphene platelets thinner than 100 nm, mostly thinner than 10 nm,and, in many cases, being single-layer graphene (also illustrated as 112in FIG. 1(B)). An NGP is composed of a graphene sheet or a plurality ofgraphene sheets with each sheet being a two-dimensional, hexagonalstructure of carbon atoms. A mass of multiple NGPs (including discretesheets/platelets of single-layer and/or few-layer graphene or grapheneoxide, 33 in FIG. 1(A)) may be made into a graphene film/paper (34 inFIG. 1(A) or 114 in FIG. 1(B)) using a film- or paper-making process.

Further alternatively, with a low-intensity shearing, graphite wormstend to be separated into the so-called expanded graphite flakes (108 inFIG. 1(B) having a thickness >100 nm. These flakes can be formed intographite paper or mat 110 using a paper- or mat-making process. Thisexpanded graphite paper or mat 110 is just a simple aggregate or stackof discrete flakes having defects, interruptions, and mis-orientationsbetween these discrete flakes.

The following examples are used to illustrate some specific detailsabout the best modes of practicing the instant invention and should notbe construed as limiting the scope of the invention.

Example 1: Various Blowing Agents and Pore-Forming (Bubble-Producing)Processes

In the field of plastic processing, chemical blowing agents are mixedinto the plastic pellets in the form of powder or pellets and dissolvedat higher temperatures. Above a certain temperature specific for blowingagent dissolution, a gaseous reaction product (usually nitrogen or CO₂)is generated, which acts as a blowing agent. However, a chemical blowingagent cannot be dissolved in a graphene material, which is a solid, notliquid. This presents a challenge to make use of a chemical blowingagent to generate pores or cells in a graphene material.

After extensive experimenting, we have discovered that practically anychemical blowing agent (e.g. in a powder or pellet form) can be used tocreate pores or bubbles in a dried layer of graphene when the first heattreatment temperature is sufficient to activate the blowing reaction.The chemical blowing agent (powder or pellets) may be dispersed in theliquid medium to become a second dispersed phase (sheets of graphenematerial being the first dispersed phase) in the suspension, which canbe deposited onto the solid supporting substrate to form a wet layer.This wet layer of graphene material may then be dried and heat treatedto activate the chemical blowing agent. After a chemical blowing agentis activated and bubbles are generated, the resulting foamed graphenestructure is largely maintained even when subsequently a higher heattreatment temperature is applied to the structure. This is quiteunexpected, indeed.

Chemical foaming agents (CFAs) can be organic or inorganic compoundsthat release gasses upon thermal decomposition. CFAs are typically usedto obtain medium- to high-density foams, and are often used inconjunction with physical blowing agents to obtain low-density foams.CFAs can be categorized as either endothermic or exothermic, whichrefers to the type of decomposition they undergo. Endothermic typesabsorb energy and typically release carbon dioxide and moisture upondecomposition, while the exothermic types release energy and usuallygenerate nitrogen when decomposed. The overall gas yield and pressure ofgas released by exothermic foaming agents is often higher than that ofendothermic types. Endothermic CFAs are generally known to decompose inthe range of 130 to 230° C. (266-446° F.), while some of the more commonexothermic foaming agents decompose around 200° C. (392° F.). However,the decomposition range of most exothermic CFAs can be reduced byaddition of certain compounds. The activation (decomposition)temperatures of CFAs fall into the range of our heat treatmenttemperatures. Examples of suitable chemical blowing agents includesodium bi-carbonate (baking soda), hydrazine, hydrazide,azodicarbonamide (exothermic chemical blowing agents), nitroso compounds(e.g. N,N-Dinitroso pentamethylene tetramine), hydrazine derivatives(e.g. 4. 4′-Oxybis (benzenesulfonyl hydrazide) and Hydrazodicarbonamide), and hydrogen carbonate (e.g. Sodium hydrogen carbonate).These are all commercially available in plastics industry.

In the production of foamed plastics, physical blowing agents aremetered into the plastic melt during foam extrusion or injection moldedfoaming, or supplied to one of the precursor materials duringpolyurethane foaming. It has not been previously known that a physicalblowing agent can be used to create pores in a graphene material, whichis in a solid state (not melt). We have surprisingly observed that aphysical blowing agent (e.g. CO₂ or N₂) can be injected into the streamof graphene suspension prior to being coated or cast onto the supportingsubstrate. This would result in a foamed structure even when the liquidmedium (e.g. water and/or alcohol) is removed. The dried layer ofgraphene material is capable of maintaining a controlled amount of poresor bubbles during liquid removal and subsequent heat treatments.

Technically feasible blowing agents include Carbon dioxide (CO₂),Nitrogen (N₂), Isobutane (C₄H₁₀), Cyclopentane (C₅H₁₀), Isopentane(C₅H₁₂), CFC-11 (CFCI₃), HCFC-22 (CHF₂CI), HCFC-142b (CF₂CICH₃), andHCFC-134a (CH₂FCF₃). However, in selecting a blowing agent,environmental safety is a major factor to consider. The MontrealProtocol and its influence on consequential agreements pose a greatchallenge for the producers of foam. Despite the effective propertiesand easy handling of the formerly applied chlorofluorocarbons, there wasa worldwide agreement to ban these because of their ozone depletionpotential (ODP). Partially halogenated chlorofluorocarbons are also notenvironmentally safe and therefore already forbidden in many countries.The alternatives are hydrocarbons, such as isobutane and pentane, andthe gases such as CO₂ and nitrogen.

Except for those regulated substances, all the blowing agents recitedabove have been tested in our experiments. For both physical blowingagents and chemical blowing agents, the blowing agent amount introducedinto the suspension is defined as a blowing agent-to-graphene materialweight ratio, which is typically from 0/1.0 to 1.0/1.0.

Example 2: Preparation of Discrete Nano Graphene Platelets (NGPs) whichare GO Sheets

Chopped graphite fibers with an average diameter of 12 μm and naturalgraphite particles were separately used as a starting material, whichwas immersed in a mixture of concentrated sulfuric acid, nitric acid,and potassium permanganate (as the chemical intercalate and oxidizer) toprepare graphite intercalation compounds (GICs). The starting materialwas first dried in a vacuum oven for 24 h at 80° C. Then, a mixture ofconcentrated sulfuric acid, fuming nitric acid, and potassiumpermanganate (at a weight ratio of 4:1:0.05) was slowly added, underappropriate cooling and stirring, to a three-neck flask containing fibersegments. After 5-16 hours of reaction, the acid-treated graphite fibersor natural graphite particles were filtered and washed thoroughly withdeionized water until the pH level of the solution reached 6. Afterbeing dried at 100° C. overnight, the resulting graphite intercalationcompound (GIC) or graphite oxide fiber was re-dispersed in water and/oralcohol to form a slurry.

In one sample, five grams of the graphite oxide fibers were mixed with2,000 ml alcohol solution consisting of alcohol and distilled water witha ratio of 15:85 to obtain a slurry mass. Then, the mixture slurry wassubjected to ultrasonic irradiation with a power of 200 W for variouslengths of time. After 20 minutes of sonication, GO fibers wereeffectively exfoliated and separated into thin graphene oxide sheetswith oxygen content of approximately 23%-31% by weight. The resultingsuspension contains GO sheets being suspended in water. A chemicalblowing agent (hydrazo dicarbonamide) was added to the suspension justprior to casting.

The resulting suspension was then cast onto a glass surface using adoctor's blade to exert shear stresses, inducing GO sheet orientations.The resulting GO coating films, after removal of liquid, have athickness that can be varied from approximately 5 to 500 μm (preferablyand typically from 10 μm to 50 μm).

For making a graphene foam specimen, the GO coating film was thensubjected to heat treatments that typically involve an initial thermalreduction temperature of 80-350° C. for 1-8 hours, followed byheat-treating at a second temperature of 1,500-2,850° C. for 0.5 to 5hours. It may be noted that we have found it essential to apply acompressive stress to the coating film sample while being subjected tothe first heat treatment. This compress stress seems to have helpedmaintain good contacts between the graphene sheets so that chemicalmerging and linking between graphene sheets can occur while pores arebeing formed. Without such a compressive stress, the heat-treated filmis typically excessively porous with constituent graphene sheets in thepore walls being very poorly oriented and incapable of chemical mergingand linking with one another. As a result, the thermal conductivity,electrical conductivity, and mechanical strength of the graphene foamare severely compromised.

Example 3: Preparation of Single-Layer Graphene Sheets from Meso-CarbonMicro-Beads (MCMBs)

Meso-carbon microbeads (MCMBs) were supplied from China Steel ChemicalCo., Kaohsiung, Taiwan. This material has a density of about 2.24 g/cm³with a median particle size of about 16 μm. MCMB (10 grams) wereintercalated with an acid solution (sulfuric acid, nitric acid, andpotassium permanganate at a ratio of 4:1:0.05) for 48-96 hours. Uponcompletion of the reaction, the mixture was poured into deionized waterand filtered. The intercalated MCMBs were repeatedly washed in a 5%solution of HCl to remove most of the sulphate ions. The sample was thenwashed repeatedly with deionized water until the pH of the filtrate wasno less than 4.5. The slurry was then subjected ultrasonication for10-100 minutes to produce GO suspensions. TEM and atomic forcemicroscopic studies indicate that most of the GO sheets weresingle-layer graphene when the oxidation treatment exceeded 72 hours,and 2- or 3-layer graphene when the oxidation time was from 48 to 72hours.

The GO sheets contain oxygen proportion of approximately 35%-47% byweight for oxidation treatment times of 48-96 hours. GO sheets weresuspended in water. Baking soda (5-20% by weight), as a chemical blowingagent, was added to the suspension just prior to casting. The suspensionwas then cast onto a glass surface using a doctor's blade to exert shearstresses, inducing GO sheet orientations. Several samples were cast,some containing a blowing agent and some not. The resulting GO films,after removal of liquid, have a thickness that can be varied fromapproximately 10 to 500 μm.

The several sheets of the GO film, with or without a blowing agent, werethen subjected to heat treatments that involve an initial (first)thermal reduction temperature of 80-500° C. for 1-5 hours. This firstheat treatment generated a graphene foam. However, the graphene domainsin the foam wall can be further perfected (re-graphitized to become moreordered or having a higher degree of crystallinity and larger lateraldimensions of graphene planes, longer than the original graphene sheetdimensions due to chemical merging) if the foam is followed byheat-treating at a second temperature of 1,500-2,850° C.

Example 4: Preparation of Pristine Graphene Foam (0% Oxygen)

Recognizing the possibility of the high defect population in GO sheetsacting to reduce the conductivity of individual graphene plane, wedecided to study if the use of pristine graphene sheets (non-oxidizedand oxygen-free, non-halogenated and halogen-free, etc.) can lead to agraphene foam having a higher thermal conductivity. Pristine graphenesheets were produced by using the direct ultrasonication or liquid-phaseproduction process.

In a typical procedure, five grams of graphite flakes, ground toapproximately 20 μm or less in sizes, were dispersed in 1,000 mL ofdeionized water (containing 0.1% by weight of a dispersing agent, Zonyl®FSO from DuPont) to obtain a suspension. An ultrasonic energy level of85 W (Branson 5450 Ultrasonicator) was used for exfoliation, separation,and size reduction of graphene sheets for a period of 15 minutes to 2hours. The resulting graphene sheets are pristine graphene that havenever been oxidized and are oxygen-free and relatively defect-free.There are no other non-carbon elements.

Various amounts (1%-30% by weight relative to graphene material) ofchemical bowing agents (N,N-Dinitroso pentamethylene tetramine or 4.4′-Oxybis (benzenesulfonyl hydrazide) were added to a suspensioncontaining pristine graphene sheets and a surfactant. The suspension wasthen cast onto a glass surface using a doctor's blade to exert shearstresses, inducing graphene sheet orientations. Several samples werecast, including one that was made using CO2 as a physical blowing agentintroduced into the suspension just prior to casting). The resultinggraphene films, after removal of liquid, have a thickness that can bevaried from approximately 10 to 100 μm.

The graphene films were then subjected to heat treatments that involvean initial (first) thermal reduction temperature of 80-1,500° C. for 1-5hours. This first heat treatment generated a graphene foam. Some of thepristine foam samples were then subjected to a second temperature of1,500-2,850° C. to determine if the graphene domains in the foam wallcould be further perfected (re-graphitized to become more ordered orhaving a higher degree of crystallinity).

Comparative Example 4-a: CVD Graphene Foams on Ni Foam Templates

The procedure was adapted from that disclosed in open literature: Chen,Z. et al. “Three-dimensional flexible and conductive interconnectedgraphene networks grown by chemical vapor deposition,” Nat. Mater. 10,424-428 (2011). Nickel foam, a porous structure with an interconnected3D scaffold of nickel was chosen as a template for the growth ofgraphene foam. Briefly, carbon was introduced into a nickel foam bydecomposing CH₄ at 1,000° C. under ambient pressure, and graphene filmswere then deposited on the surface of the nickel foam. Due to thedifference in the thermal expansion coefficients between nickel andgraphene, ripples and wrinkles were formed on the graphene films. Inorder to recover (separate) graphene foam, Ni frame must be etched away.Before etching away the nickel skeleton by a hot HCl (or FeCl₃)solution, a thin layer of poly(methyl methacrylate) (PMMA) was depositedon the surface of the graphene films as a support to prevent thegraphene network from collapsing during nickel etching. After the PMMAlayer was carefully removed by hot acetone, a fragile graphene foamsample was obtained. The use of the PMMA support layer is critical topreparing a free-standing film of graphene foam; only a severelydistorted and deformed graphene foam sample was obtained without thePMMA support layer. This is a tedious process that is notenvironmentally benign and is not scalable.

Comparative Example 4-b: Conventional Graphitic Foam from Pitch-BasedCarbon Foams

Pitch powder, granules, or pellets are placed in a aluminum mold withthe desired final shape of the foam. Mitsubishi ARA-24 meso-phase pitchwas utilized. The sample is evacuated to less than 1 torr and thenheated to a temperature approximately 300° C. At this point, the vacuumwas released to a nitrogen blanket and then a pressure of up to 1,000psi was applied. The temperature of the system was then raised to 800°C. This was performed at a rate of 2 degree C./min. The temperature washeld for at least 15 minutes to achieve a soak and then the furnacepower was turned off and cooled to room temperature at a rate ofapproximately 1.5 degree C./min with release of pressure at a rate ofapproximately 2 psi/min. Final foam temperatures were 630° C. and 800°C. During the cooling cycle, pressure is released gradually toatmospheric conditions. The foam was then heat treated to 1050° C.(carbonized) under a nitrogen blanket and then heat treated in separateruns in a graphite crucible to 2500° C. and 2800° C. (graphitized) inArgon.

Samples from the foam were machined into specimens for measuring thethermal conductivity. The bulk thermal conductivity ranged from 67 W/mKto 151 W/mK. The density of the samples was from 0.31-0.61 g/cm³. Whenweight is taken into account, the specific thermal conductivity of thepitch derived foam is approximately 67/0.31=216 and 151/0.61=247.5 W/mKper specific gravity (or per physical density).

The compression strength of the samples having an average density of0.51 g/cm³ was measured to be 3.6 MPa and the compression modulus wasmeasured to be 74 MPa. By contrast, the compression strength andcompressive modulus of the presently invented graphene foam samplesderived from GO having a comparable physical density are 5.7 MPa and 103MPa, respectively.

Shown in FIG. 4(A) and FIG. 5(A) are the thermal conductivity values vs.specific gravity of the GO suspension-derived foam, meso-phasepitch-derived graphite foam, and Ni foam template-assisted CVD graphenefoam. These data clearly demonstrate the following unexpected results:

-   -   1) GO-derived graphene foams produced by the presently invented        process exhibit significantly higher thermal conductivity as        compared to both meso-phase pitch-derived graphite foam and Ni        foam template-assisted CVD graphene, given the same physical        density.    -   2) This is quite surprising in view of the notion that CVD        graphene is essentially pristine graphene that has never been        exposed to oxidation and should have exhibited a much higher        thermal conductivity compared to graphene oxide (GO). GO is        known to be highly defective (having a high defect population        and, hence, low conductivity) even after the oxygen-containing        functional groups are removed via conventional thermal or        chemical reduction methods. These exceptionally high thermal        conductivity values observed with the GO-derived graphene foams        herein produced are much to our surprise.    -   3) FIG. 5(A) presents the thermal conductivity values over        comparable ranges of specific gravity values to allow for        calculation of specific conductivity (conductivity value, W/mK,        divided by physical density value, g/cm³) for all three        graphitic foam materials based on the slopes of the curves        (approximately straight lines at different segments). These        specific conductivity values enable a fair comparison of thermal        conductivity values of these three types of graphitic foams        given the same amount of solid graphitic material in each foam.        These data provide an index of the intrinsic conductivity of the        solid portion of the foam material. These data clearly indicate        that, given the same amount of solid material, the presently        invented GO-derived foam is intrinsically most conducting,        reflecting a high level of graphitic crystal perfection (larger        crystal dimensions, fewer grain boundaries and other defects,        better crystal orientation, etc.). This is also unexpected.    -   4) The specific conductivity values of the presently invented        GO- and GF-derived foam exhibit values from 250 to 500 W/mK per        unit of specific gravity; but those of the other two foam        materials are typically lower than 250 W/mK per unit of specific        gravity.

Summarized in FIG. 7 are thermal conductivity data for a series ofGO-derived graphene foams and a series of pristine graphene derivedfoams, both plotted over the final (maximum) heat treatmenttemperatures. These data indicate that the thermal conductivity of theGO foams is highly sensitive to the final heat treatment temperature(HTT). Even when the HTT is very low, clearly some type of graphenemerging or crystal perfection reactions are already activated. Thethermal conductivity increases monotonically with the final HTT. Incontrast, the thermal conductivity of pristine graphene foams remainsrelatively constant until a final HTT of approximately 2,500° C. isreached, signaling the beginning of a re-crystallization and perfectionof graphite crystals. There are no functional groups in pristinegraphene, such as —COOH in GO, that enable chemical linking of graphenesheets at relatively low HTTs. With a HTT as low as 1,250° C., GO sheetscan merge to form significantly larger graphene sheets with reducedgrain boundaries and other defects. Even though GO sheets areintrinsically more defective than pristine graphene, the presentlyinvented process enables the GO sheets to form graphene foams thatoutperform pristine graphene foams. This is another unexpected result.

Example 5: Preparation of Graphene Oxide (GO) Suspension from NaturalGraphite and of Subsequent GO Foams

Graphite oxide was prepared by oxidation of graphite flakes with anoxidizer liquid consisting of sulfuric acid, sodium nitrate, andpotassium permanganate at a ratio of 4:1:0.05 at 30° C. When naturalgraphite flakes (particle sizes of 14 μm) were immersed and dispersed inthe oxidizer mixture liquid for 48 hours, the suspension or slurryappears and remains optically opaque and dark. After 48 hours, thereacting mass was rinsed with water 3 times to adjust the pH value to atleast 3.0. A final amount of water was then added to prepare a series ofGO-water suspensions. We observed that GO sheets form a liquid crystalphase when GO sheets occupy a weight fraction >3% and typically from 5%to 15%.

By dispensing and coating the GO suspension on a polyethyleneterephthalate (PET) film in a slurry coater and removing the liquidmedium from the coated film we obtained a thin film of dried grapheneoxide. Several GO film samples were then subjected to different heattreatments, which typically include a thermal reduction treatment at afirst temperature of 100° C. to 500° C. for 1-10 hours, and at a secondtemperature of 1,500° C.-2,850° C. for 0.5-5 hours. With these heattreatments, also under a compressive stress, the GO films weretransformed into graphene foam.

Comparative Example 5-a: Graphene Foams from Hydrothermally ReducedGraphene Oxide

For comparison, a self-assembled graphene hydrogel (SGH) sample wasprepared by a one-step hydrothermal method. In a typical procedure, theSGH can be easily prepared by heating 2 mg/mL of homogeneous grapheneoxide (GO) aqueous dispersion sealed in a Teflon-lined autoclave at 180°C. for 12 h. The SGH containing about 2.6% (by weight) graphene sheetsand 97.4% water has an electrical conductivity of approximately 5×10⁻³S/cm. Upon drying and heat treating at 1,500° C., the resulting graphenefoam exhibits an electrical conductivity of approximately 1.5×10⁻¹ S/cm,which is 2 times lower than those of the presently invented graphenefoams produced by heat treating at the same temperature.

Comparative Example 5-b: Plastic Bead Template-Assisted Formation ofReduced Graphene Oxide Foams

A hard template-directed ordered assembly for a macro-porous bubbledgraphene film (MGF) was prepared. Mono-disperse poly methyl methacrylate(PMMA) latex spheres were used as the hard templates. The GO liquidcrystal prepared in Example 5 was mixed with a PMMA spheres suspension.Subsequent vacuum filtration was then conducted to prepare the assemblyof PMMA spheres and GO sheets, with GO sheets wrapped around the PMMAbeads. A composite film was peeled off from the filter, air dried andcalcinated at 800° C. to remove the PMMA template and thermally reduceGO into RGO simultaneously. The grey free-standing PMMA/GO film turnedblack after calcination, while the graphene film remained porous.

FIG. 4(B) and FIG. 5(B) show the thermal conductivity values of thepresently invented GO suspension-derived foam, GO foam produced viasacrificial plastic bead template-assisted process, and hydrothermallyreduced GO graphene foam. Most surprisingly, given the same starting GOsheets, the presently invented process produces the highest-performinggraphene foams. Electrical conductivity data summarized in FIG. 4(C) arealso consistent with this conclusion. These data further support thenotion that, given the same amount of solid material, the presentlyinvented GO suspension deposition (with stress-induced orientation) andsubsequent heat treatments give rise to a graphene foam that isintrinsically most conducting, reflecting a highest level of graphiticcrystal perfection (larger crystal dimensions, fewer grain boundariesand other defects, better crystal orientation, etc. along the porewalls).

It is of significance to point out that all the prior art processes forproducing graphite foams or graphene foams appear to providemacro-porous foams having a physical density in the range ofapproximately 0.2-0.6 g/cm³ only with pore sizes being typically toolarge (e.g. from 20 to 300 μm) for most of the intended applications. Incontrast, the instant invention provides processes that generategraphene foams having a density that can be as low as 0.01 g/cm³ and ashigh as 1.7 g/cm³. The pore sizes can be varied between meso-scaled(2-50 nm) up to macro-scaled (1-500 μm) depending upon the contents ofnon-carbon elements and the amount/type of blowing agent used. Thislevel of flexibility and versatility in designing various types ofgraphene foams is unprecedented and un-matched by any prior art process.

Example 6: Preparation of Graphene Foams from Graphene Fluoride

Several processes have been used by us to produce GF, but only oneprocess is herein described as an example. In a typical procedure,highly exfoliated graphite (HEG) was prepared from intercalated compoundC₂F.xClF₃. HEG was further fluorinated by vapors of chlorine trifluorideto yield fluorinated highly exfoliated graphite (FHEG). Pre-cooledTeflon reactor was filled with 20-30 mL of liquid pre-cooled ClF₃, thereactor was closed and cooled to liquid nitrogen temperature. Then, nomore than 1 g of HEG was put in a container with holes for ClF₃ gas toaccess and situated inside the reactor. In 7-10 days a gray-beigeproduct with approximate formula C₂F was formed.

Subsequently, a small amount of FHEG (approximately 0.5 mg) was mixedwith 20-30 mL of an organic solvent (methanol, ethanol, 1-propanol,2-propanol, 1-butanol, tert-butanol, isoamyl alcohol) and subjected toan ultrasound treatment (280 W) for 30 min, leading to the formation ofhomogeneous yellowish dispersions. Five minutes of sonication was enoughto obtain a relatively homogenous dispersion, but longer sonicationtimes ensured better stability. Upon casting on a glass surface with thesolvent removed, the dispersion became a brownish film formed on theglass surface. When GF films were heat-treated, fluorine was released asgases that helped to generate pores in the film. In some samples, aphysical blowing agent (N₂ gas) was injected into the wet GF film whilebeing cast. These samples exhibit much higher pore volumes or lower foamdensities. Without using a blowing agent, the resulting graphenefluoride foams exhibit physical densities from 0.35 to 1.38 g/cm³. Whena blowing agent was used (blowing agent/GF weight ratio from 0.5/1 to0.05/1), a density from 0.02 to 0.35 g/cm³ was obtained. Typicalfluorine contents are from 0.001% (HTT=2,500° C.) to 4.7% (HTT=350° C.),depending upon the final heat treatment temperature involved.

FIG. 6 presents a comparison in thermal conductivity values of thegraphene foam samples derived from GO and GF (graphene fluoride),respectively, as a function of the specific gravity. It appears that theGF foams, in comparison with GO foams, exhibit higher thermalconductivity values at comparable specific gravity values. Both deliverimpressive heat-conducting capabilities, being the best among all knownfoamed materials.

Example 7: Preparation of Graphene Foams from Nitrogenated Graphene

Graphene oxide (GO), synthesized in Example 2, was finely ground withdifferent proportions of urea and the pelletized mixture heated in amicrowave reactor (900 W) for 30 s. The product was washed several timeswith deionized water and vacuum dried. In this method graphene oxidegets simultaneously reduced and doped with nitrogen. The productsobtained with graphene:urea mass ratios of 1:0.5, 1:1 and 1:2 aredesignated as NGO-1, NGO-2 and NGO-3 respectively and the nitrogencontents of these samples were 14.7, 18.2 and 17.5 wt % respectively asfound by elemental analysis. These nitrogenated graphene sheets remaindispersible in water. The resulting suspensions were then cast, dried,and heat-treated initially at 200-350° C. as a first heat treatmenttemperature and subsequently treated at a second temperature of 1,500°C. The resulting nitrogenated graphene foams exhibit physical densitiesfrom 0.45 to 1.28 g/cm³. Typical nitrogen contents of the foams are from0.01% (HTT=1,500° C.) to 5.3% (HTT=350° C.), depending upon the finalheat treatment temperature involved.

Example 8: Characterization of Various Graphene Foams and ConventionalGraphite Foam

The internal structures (crystal structure and orientation) of severaldried GO layers, and the heat-treated films at different stages of heattreatments were investigated using X-ray diffraction. The X-raydiffraction curve of natural graphite typically exhibits a peak atapproximately 20=26°, corresponds to an inter-graphene spacing (d₀₀₂) ofapproximately 0.3345 nm. Upon oxidation, the resulting GO shows an X-raydiffraction peak at approximately 2θ=12°, which corresponds to aninter-graphene spacing (d₀₀₂) of approximately 0.7 nm. With some heattreatment at 150° C., the dried GO compact exhibits the formation of ahump centered at 22°, indicating that it has begun the process ofdecreasing the inter-graphene spacing due to the beginning of chemicallinking and ordering processes. With a heat treatment temperature of2,500° C. for one hour, the d₀₀₂ spacing has decreased to approximately0.336, close to 0.3354 nm of a graphite single crystal.

With a heat treatment temperature of 2,750° C. for one hour, the d₀₀₂spacing is decreased to approximately to 0.3354 nm, identical to that ofa graphite single crystal. In addition, a second diffraction peak with ahigh intensity appears at 2θ=55° corresponding to X-ray diffraction from(004) plane. The (004) peak intensity relative to the (002) intensity onthe same diffraction curve, or the I(004)/I(002) ratio, is a goodindication of the degree of crystal perfection and preferred orientationof graphene planes. The (004) peak is either non-existing or relativelyweak, with the I(004)/I(002) ratio <0.1, for all graphitic materialsheat treated at a temperature lower than 2,800° C. The I(004)/I(002)ratio for the graphitic materials heat treated at 3,000-3,250° C. (e,g,highly oriented pyrolytic graphite, HOPG) is in the range of 0.2-0.5. Incontrast, a graphene foam prepared with a final HTT of 2,750° C. for onehour exhibits a I(004)/I(002) ratio of 0.78 and a Mosaic spread value of0.21, indicating a practically perfect graphene single crystal with agood degree of preferred orientation.

The “mosaic spread” value is obtained from the full width at halfmaximum of the (002) reflection in an X-ray diffraction intensity curve.This index for the degree of ordering characterizes the graphite orgraphene crystal size (or grain size), amounts of grain boundaries andother defects, and the degree of preferred grain orientation. A nearlyperfect single crystal of graphite is characterized by having a mosaicspread value of 0.2-0.4. Some of our graphene foams have a mosaic spreadvalue in this range of 0.2-0.4 when produced using a final heattreatment temperature no less than 2,500° C.

The inter-graphene spacing values of both the GO suspension-derivedsamples obtained by heat treating at various temperatures over a widetemperature range are summarized in FIG. 8(A). Corresponding oxygencontent values in the GO suspension-derived unitary graphene layer areshown in FIG. 8(B).

It is of significance to point out that a heat treatment temperature aslow as 500° C. is sufficient to bring the average inter-graphene spacingin GO sheets along the pore walls to below 0.4 nm, getting closer andcloser to that of natural graphite or that of a graphite single crystal.The beauty of this approach is the notion that this GO suspensionstrategy has enabled us to re-organize, re-orient, and chemically mergethe planar graphene oxide molecules from originally different graphiteparticles or graphene sheets into a unified structure with all thegraphene planes now being larger in lateral dimensions (significantlylarger than the length and width of the graphene planes in the originalgraphite particles). A potential chemical linking mechanism isillustrated in FIG. 3. This has given rise to exceptional thermalconductivity and electrical conductivity values.

In conclusion, we have successfully developed an absolutely new, novel,unexpected, and patently distinct class of highly conducting graphenefoam materials and related processes of production. The chemicalcomposition (% of oxygen, fluorine, and other non-carbon elements),structure (crystal perfection, grain size, defect population, etc),crystal orientation, morphology, process of production, and propertiesof this new class of foam materials are fundamentally different andpatently distinct from meso-phase pitch-derived graphite foam, CVDgraphene-derived foam, and graphene foams from hydrothermal reduction ofGO, and sacrificial bead template-assisted RGO foam. The thermalconductivity, electrical conductivity, elastic modulus, and flexuralstrength exhibited by the presently invented foam materials are muchhigher than what prior art foam materials.

We claim:
 1. A solid graphene foam composed of multiple pores and porewalls, wherein said pore walls contain a pristine graphene materialhaving essentially zero % of non-carbon elements, or a non-pristinegraphene material having 0.001% to 5% by weight of non-carbon elementswherein said non-pristine graphene is selected from graphene oxide,reduced graphene oxide, graphene fluoride, graphene chloride, graphenebromide, graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, or a combination thereof, and saidsolid graphene foam has a density from 0.01 g/cm³ to 1.7 g/cm³, aspecific surface area from 50 m²/g to 3,000 m²/g, a thermal conductivityof at least 200 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 2,000 S/cm per unit of specific gravity. 2.The solid graphene foam of claim 1, wherein said pore walls containstacked graphene planes having an inter-plane spacing d₀₀₂ from 0.3354nm to 0.36 nm as measured by X-ray diffraction.
 3. The solid graphenefoam of claim 1, wherein said pore walls contain a pristine graphene andsaid solid graphene foam has a density from 0.5 g/cm³ to 1.7 g/cm³ or anaverage pore size from 2 nm to 50 nm.
 4. The solid graphene foam ofclaim 1, wherein said pore walls contain a non-pristine graphenematerial selected from the group consisting of graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, and combinations thereof, andwherein said solid graphene foam contains a content of non-carbonelements in the range of 0.01% to 2.0% by weight.
 5. The solid graphenefoam of claim 1, wherein said pore walls contain graphene fluoride andsaid solid graphene foam contains a fluorine content from 0.01% to 2.0%by weight.
 6. The solid graphene foam of claim 1, wherein said porewalls contain graphene oxide and said solid graphene foam contains anoxygen content from 0.01% to 2.0% by weight.
 7. The solid graphene foamof claim 1, wherein said solid graphene foam has a specific surface areafrom 200 m²/g to 2,000 m²/g or a density from 0.1 g/cm³ to 1.5 g/cm³. 8.The solid graphene foam of claim 1, wherein said non-carbon elementsinclude an element selected from oxygen, fluorine, chlorine, bromine,iodine, nitrogen, hydrogen, or boron.
 9. The solid graphene foam ofclaim 1, which is in a continuous-length roll sheet form having athickness no greater than 200 μm and a length of at least 2 meters andis produced by a roll-to-roll process.
 10. The solid graphene foam ofclaim 1, wherein said graphene foam has an oxygen content or non-carboncontent less than 1% by weight, and said pore walls have aninter-graphene spacing less than 0.35 nm, a thermal conductivity of atleast 250 W/mK per unit of specific gravity, and/or an electricalconductivity no less than 2,500 S/cm per unit of specific gravity. 11.The solid graphene foam of claim 1, wherein said graphene foam has anoxygen content or non-carbon content less than 0.01% by weight and saidpore walls contain stacked graphene planes having an inter-graphenespacing less than 0.34 nm, a thermal conductivity of at least 300 W/mKper unit of specific gravity, and/or an electrical conductivity no lessthan 3,000 S/cm per unit of specific gravity.
 12. The solid graphenefoam of claim 1, wherein said graphene foam has an oxygen content ornon-carbon content no greater than 0.01% by weight and said pore wallscontain stacked graphene planes having an inter-graphene spacing lessthan 0.336 nm, a mosaic spread value no greater than 0.7, a thermalconductivity of at least 350 W/mK per unit of specific gravity, and/oran electrical conductivity no less than 3,500 S/cm per unit of specificgravity.
 13. A solid graphene foam composed of multiple pores and porewalls, wherein said pore walls are comprised of stacked graphene planeshaving an inter-graphene spacing less than 0.336 nm, a mosaic spreadvalue no greater than 0.4, and are selected from the group comprising apristine graphene material having essentially zero % of non-carbonelements, a non-pristine graphene material having 0.001% to 5% by weightof non-carbon elements, and combinations thereof, wherein saidnon-pristine graphene is selected from graphene oxide, reduced grapheneoxide, graphene fluoride, graphene chloride, graphene bromide, grapheneiodide, hydrogenated graphene, nitrogenated graphene, chemicallyfunctionalized graphene, and combinations thereof, and said solidgraphene foam has a density from 0.01 g/cm³ to 1.7 g/cm³, a specificsurface area from 50 m²/g to 3,000 m²/g, a thermal conductivity of atleast 400 W/mK per unit of specific gravity, and an electricalconductivity no less than 4,000 S/cm per unit of specific gravity. 14.The solid graphene foam of claim 1, wherein the pore walls containstacked graphene planes having an inter-graphene spacing less than 0.337nm and a mosaic spread value less than 1.0.
 15. The solid graphene foamof claim 1, wherein the solid graphene foam exhibits a degree ofgraphitization no less than 80% and/or a mosaic spread value less than0.4.
 16. The solid graphene foam of claim 1, wherein the solid graphenefoam exhibits a degree of graphitization no less than 90% and/or amosaic spread value no greater than 0.4.
 17. The solid graphene foam ofclaim 1, wherein said pore walls contain a 3D network of interconnectedgraphene planes.
 18. The solid graphene foam of claim 1, wherein saidsolid graphene foam contains mesoscaled pores having a pore size from 2nm to 50 nm.
 19. A process for producing the solid graphene foam ofclaim 1, said process comprising: (a) preparing a graphene dispersionhaving multiple sheets of a starting graphene material dispersed in aliquid medium, wherein said starting graphene material is selected froma pristine graphene or a non-pristine graphene material, having acontent of non-carbon elements greater than 2% by weight, selected fromgraphene oxide, reduced graphene oxide, graphene fluoride, graphenechloride, graphene bromide, graphene iodide, hydrogenated graphene,nitrogenated graphene, chemically functionalized graphene, or acombination thereof and wherein said dispersion contains an optionalblowing agent having a blowing agent-to-graphene material weight ratiofrom 0/1.0 to 1.0/1.0; (b) dispensing and depositing said graphenedispersion onto a surface of a supporting substrate to form a wet layerof graphene material, wherein said dispensing and depositing procedureincludes subjecting said graphene dispersion to an orientation-inducingstress; (c) partially or completely removing said liquid medium from thewet layer of graphene material to form a dried layer of graphenematerial; and (d) heat treating the dried layer of graphene material ata first heat treatment temperature from 80° C. to 3,200° C. at a desiredheating rate sufficient to induce volatile gas molecules from saidnon-carbon elements or to activate said blowing agent for producing saidsheet of solid graphene foam.
 20. The process of claim 19, furtherincluding a step of heat-treating the solid graphene foam at a secondheat treatment temperature higher than said first heat treatmenttemperature for a length of time sufficient for obtaining a graphenefoam wherein said pore walls contain stacked graphene planes having aninter-plane spacing d₀₀₂ from 0.3354 nm to 0.36 nm and a content ofnon-carbon elements less than 2% by weight.
 21. The process of claim 19,wherein said graphene material contains pristine graphene and saiddispersion contains a blowing agent having a blowing agent-to-pristinegraphene weight ratio from 0.01/1.0 to 1.0/1.0.
 22. The process of claim19, wherein said blowing agent is a physical blowing agent, a chemicalblowing agent, a mixture thereof, a dissolution-and-leaching agent, or amechanically introduced blowing agent.
 23. The process of claim 19,wherein said graphene material is selected from the group ofnon-pristine graphene materials consisting of graphene oxide, reducedgraphene oxide, graphene fluoride, graphene chloride, graphene bromide,graphene iodide, hydrogenated graphene, nitrogenated graphene,chemically functionalized graphene, and combinations thereof, andwherein the first heat treatment temperature is less than 2,500° C. andsaid solid graphene foam contains a content of non-carbon elements from0.01% to 2.0% by weight.
 24. The process of claim 20, wherein saidgraphene material is selected from the group of non-pristine graphenematerials consisting of graphene oxide, reduced graphene oxide, graphenefluoride, graphene chloride, graphene bromide, graphene iodide,hydrogenated graphene, nitrogenated graphene, chemically functionalizedgraphene, and combinations thereof, and wherein both the first andsecond heat treatment temperatures are less than 2,500° C. and saidsolid graphene foam contains a content of non-carbon elements in therange of 0.01% to 2.0% by weight.
 25. The process of claim 19, whereinsaid graphene material in said solid graphene foam contains structuraldefects induced during said step (d) of heat treating.
 26. The processof claim 19, wherein said liquid medium consists of water and/or analcohol.
 27. The process of claim 19, which is a roll-to-roll processwherein said steps (b) and (c) include feeding said supporting substratefrom a feeder roller to a deposition zone, continuously orintermittently depositing said graphene dispersion onto a surface ofsaid supporting substrate to form said wet layer of graphene materialthereon, drying said wet layer of graphene material to form the driedlayer of graphene material, and collecting said dried layer of graphenematerial deposited on said supporting substrate on a collector roller.28. The process of claim 19, wherein said first heat treatmenttemperature is from 100° C. to 1,500° C.
 29. The process of claim 20,wherein said second heat treatment temperature includes at least atemperature selected from (A) 300-1,500° C., (B) 1,500-2,100° C., and/or(C) 2,100-3,200° C.
 30. The process of claim 20, wherein said secondheat treatment temperature includes a temperature in the range of300-1,500° C. for at least 1 hour and then a temperature in the range of1,500-3,200° C. for at least 1 hour.
 31. The process of claim 19,wherein said non-carbon elements include an element selected fromoxygen, fluorine, chlorine, bromine, iodine, nitrogen, hydrogen, orboron.
 32. The process of claim 19, wherein said step (d) of heattreating the dried layer of graphene material at a first heat treatmenttemperature is conducted under a compressive stress.
 33. The process ofclaim 20, further comprising a compression step to reduce a thickness, apore size, or a porosity level of said solid graphene foam.
 34. Theprocess of claim 19, wherein said graphene dispersion contains agraphene oxide dispersion prepared by immersing a graphitic material ina powder or fibrous form in an oxidizing liquid in a reaction vessel ata reaction temperature for a length of time sufficient to obtain saidgraphene dispersion wherein said graphitic material is selected fromnatural graphite, artificial graphite, mesophase carbon, mesophasepitch, mesocarbon micro-bead, soft carbon, hard carbon, coke, carbonfiber, carbon nanofiber, carbon nanotube, or a combination thereof andwherein said graphene oxide has an oxygen content no less than 5% byweight.
 35. The process of claim 20, wherein said first and/or secondheat treatment temperature contains a temperature in the range of 300°C.-1,500° C. and the graphene foam has an oxygen content or non-carboncontent less than 1%, and pore walls having an inter-graphene spacingless than 0.35 nm, a thermal conductivity of at least 250 W/mK per unitof specific gravity to 500 W/mK per unit of specific gravity, and/or anelectrical conductivity from 2,500 S/cm per unit of specific gravity.36. The process of claim 20, wherein said first and/or second heattreatment temperature contains a temperature in the range of 1,500°C.-2,100° C. and the graphene foam has an oxygen content or non-carboncontent less than 0.01%, pore walls having an inter-graphene spacingless than 0.34 nm, a thermal conductivity of at least 300 W/mK per unitof specific gravity to 500 W/mK per unit of specific gravity, and/or anelectrical conductivity from 3,000 S/cm per unit of specific gravity.37. The process of claim 20, wherein said first and/or second heattreatment temperature contains a temperature greater than 2,100° C. andthe graphene foam has an oxygen content or non-carbon content no greaterthan 0.001%, pore walls having an inter-graphene spacing less than 0.336nm, a mosaic spread value no greater than 0.7, a thermal conductivity ofat least 350 W/mK per unit of specific gravity to 500 W/mK per unit ofspecific gravity, and/or an electrical conductivity greater than 3,500S/cm per unit of specific gravity.
 38. A process for producing a solidgraphene foam composed of multiple pores and pore walls, said processcomprising: (a) preparing a graphene dispersion having multiple sheetsof a starting graphene material dispersed in a liquid medium, furthercomprising an optional blowing agent having a blowing agent-to-graphenematerial weight ratio from 0/1.0 to 1.0/1.0; (b) dispensing anddepositing said graphene dispersion onto a surface of a supportingsubstrate to form a wet layer of graphene material, wherein saiddispensing and depositing procedure includes subjecting said graphenedispersion to an orientation-inducing stress; (c) partially orcompletely removing said liquid medium from the wet layer of graphenematerial to form a dried layer of graphene material; and (d) heattreating the dried layer of graphene material at a first heat treatmenttemperature from 80° C. to 3,200° C. at a desired heating ratesufficient to induce volatile gas molecules from said non-carbonelements or to activate said blowing agent for producing said sheet ofsolid graphene foam; (e) and optionally heat treating the dried layer ofgraphene material at a second heat treatment temperature from 80° C. to3,200° C.; wherein said first and/or second heat treatment temperaturecontains a temperature no less than 2,500° C. and wherein said porewalls contain a pristine graphene material having essentially zeropercent of non-carbon elements, or a non-pristine graphene materialhaving 0.001% to 5% by weight of non-carbon elements wherein saidnon-pristine graphene is selected from graphene oxide, reduced grapheneoxide, graphene fluoride, graphene chloride, graphene bromide, grapheneiodide, hydrogenated graphene, nitrogenated graphene, chemicallyfunctionalized graphene, or a combination thereof, wherein said porewalls contain stacked graphene planes having an inter-graphene spacingless than 0.336 nm, and a mosaic spread value no greater than 0.4, andthe solid graphene foam has a density from 0.01 g/cm³ to 1.7 g/cm³, aspecific surface area from 50 m²/g to 3,000 m²/g, a thermal conductivitygreater than 400 W/mK per unit of specific gravity, and/or an electricalconductivity greater than 4,000 S/cm per unit of specific gravity.
 39. Aroll-to-roll process for producing a continuous-length sheet of thesolid graphene foam of claim 1, said process comprising: (a) preparing agraphene dispersion having a graphene material dispersed in a liquidmedium, wherein said dispersion contains a blowing agent; (b)continuously or intermittently dispensing and depositing said graphenedispersion onto a surface of a supporting substrate to form a wet layerof graphene material, wherein said supporting substrate is a continuousthin film supplied from a feeder roller and collected on a collectorroller; (c) partially or completely removing said liquid medium from thewet layer of graphene material to form a dried layer of graphene; andheat treating the dried layer of graphene material at a first heattreatment temperature from 100° C. to 3,000° C. at a desired heatingrate sufficient to activate said blowing agent for producing said solidgraphene foam.